Property, Plant and Equipment [Abstract] Capitalized costs related to software, equipment, and leaseholds Capitalized Costs, Additions To Software, Equipment, Leasehold Capitalized Costs, Additions To Software, Equipment, Leasehold Depreciation and amortization Depreciation, Depletion and Amortization Reinsurance Disclosures [Abstract] Reinsurance Reinsurance [Text Block] Statement of Comprehensive Income [Abstract] Change in unrealized investment gains (losses), tax amount Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Reclassification adjustment from AOCI for sale of securities, tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Income Tax Disclosure [Abstract] Income Tax Contingency [Table] Income Tax Contingency [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] ASU No. 2018-02 Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Domestic Tax Authority Domestic Tax Authority [Member] State and Local Jurisdiction State and Local Jurisdiction [Member] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Domain] Income Tax Authority, Name [Domain] IRS Internal Revenue Service (IRS) [Member] Expiration Period [Axis] Expiration Period [Axis] Expiration Period [Axis] Expiration Period [Domain] Expiration Period [Domain] [Domain] for Expiration Period [Axis] Annual Limitation Through Year Two Annual Limitation Through Year Two [Member] Annual Limitation Through Year Two [Member] Annual Limitations After Year Two Annual Limitations After Year Two [Member] Annual Limitations After Year Two [Member] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Income tax expense (benefit) Income Tax Expense (Benefit) Provisional income tax expense related to Tax Cuts and Jobs Act of 2017 Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Provisional Income Tax Expense (Benefit) Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Provisional Income Tax Expense (Benefit) Decrease in deferred tax asset valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Net deferred tax liability Deferred Tax Liabilities, Net Net deferred tax asset Deferred Tax Assets, Net Decrease to retained earnings upon adoption of new accounting standard Cumulative Effect of New Accounting Principle in Period of Adoption Stranded tax in AOCI related to available-for-sale fixed income holdings Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-For-Sale, Stranded Tax Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-For-Sale, Stranded Tax Operating loss carryforwards Operating Loss Carryforwards Loss carry forwards subject to expiration Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration Valuation allowance Deferred Tax Assets, Valuation Allowance Unrecognized tax benefits reserve Unrecognized Tax Benefits Condensed Financial Information Disclosure [Abstract] Condensed Income Statement [Table] Condensed Income Statement [Table] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Parent Parent Company [Member] Condensed Income Statements, Captions [Line Items] Condensed Income Statements, Captions [Line Items] Debt Disclosure [Abstract] Warrants Line of Credit and Related Warrants [Text Block] Line of Credit and Related Warrants [Text Block] Statement [Table] Statement [Table] Statement [Line Items] Statement [Line Items] Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common stock, issued (in shares) Common Stock, Shares, Issued Common stock, outstanding (in shares) Common Stock, Shares, Outstanding Common stock, authorized (in shares) Common Stock, Shares Authorized Schedule of Equipment and Software Property, Plant and Equipment [Table Text Block] Accounting Policies [Abstract] Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Insurance Premium Revenue Recognition Insurance Premiums Revenue Recognition, Policy [Policy Text Block] Reserve for Insurance Claims and Claims Expenses Liability Reserve Estimate, Policy [Policy Text Block] Investments Marketable Securities, Policy [Policy Text Block] Deferred Policy Acquisition Costs Deferred Policy Acquisition Costs, Policy [Policy Text Block] Premium Deficiency Reserves Premium Deficiency Reserves Policy [Policy Text Block] Premium Deficiency Reserves Policy [Policy Text Block] Reinsurance Reinsurance Accounting Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Warrants Warrants Policy [Policy Text Block] Warrants Policy [Policy Text Block] Share-Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Earnings per Share Earnings Per Share, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Software and Equipment Property, Plant and Equipment, Policy [Policy Text Block] Business Combinations, Goodwill and Intangible Assets Goodwill and Intangible Assets, Policy [Policy Text Block] Premiums Receivable Premiums Receivable, Basis of Accounting, Policy [Policy Text Block] Variable interest entity Consolidation, Variable Interest Entity, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Identifiable Intangible Assets and Goodwill Schedule of Indefinite-Lived Intangible Assets [Table Text Block] Software and Equipment Property, Plant and Equipment Disclosure [Text Block] Financial Information of Registrant Parent Company Only Condensed Financial Information of Parent Company Only Disclosure [Text Block] Insurance [Abstract] Reserves for Insurance Claims and Claim Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Statement of Financial Position [Abstract] Assets Assets [Abstract] Fixed maturities, available-for-sale, at fair value (amortized cost of $924,987 and $713,859 as of December 31, 2018 and December 31, 2017, respectively) Debt Securities, Available-for-sale Cash and cash equivalents (including restricted cash of $1,414 and $0 as of December 31, 2018 and December 31, 2017, respectively) Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Premiums receivable Premiums Receivable, Net Accrued investment income Accrued Investment Income Receivable Prepaid expenses Prepaid Expense Deferred policy acquisition costs, net Deferred Policy Acquisition Costs Software and equipment, net Property, Plant and Equipment, Net Intangible assets and goodwill Intangible Assets, Net (Including Goodwill) Prepaid reinsurance premiums Prepaid Reinsurance Premiums Deferred tax asset, net Deferred Income Tax Assets, Net Other assets Other Assets Total assets Assets Liabilities Liabilities [Abstract] Term loan Long-term Debt Unearned premiums Unearned Premiums Accounts payable and accrued expenses Accounts Payable and Accrued Liabilities Reserve for insurance claims and claim expenses Liability for Claims and Claims Adjustment Expense Reinsurance funds withheld Funds Held under Reinsurance Agreements, Liability Deferred ceding commission Deferred Ceding Commission Deferred Ceding Commission Warrant liability, at fair value Warrants and Rights Outstanding Deferred tax liability, net Deferred Income Tax Liabilities, Net Total liabilities Liabilities Commitments and contingencies (see Note 14) Commitments and Contingencies Shareholders' equity Stockholders' Equity Attributable to Parent [Abstract] Common stock - class A shares, $0.01 par value; 66,318,849 and 60,517,512 shares issued and outstanding as of December 31, 2018 and December 31, 2017, respectively (250,000,000 shares authorized) Common Stock, Value, Outstanding Additional paid-in capital Additional Paid in Capital Accumulated other comprehensive loss, net of tax Accumulated Other Comprehensive Income (Loss), Net of Tax Retain earnings (accumulated deficit) Retained Earnings (Accumulated Deficit) Total shareholders' equity Stockholders' Equity Attributable to Parent Total liabilities and shareholders' equity Liabilities and Equity Summary of Accounting Principles Significant Accounting Policies [Text Block] Schedule of Combined Statutory Net Loss, Statutory Surplus, Contingency Reserve and RTC Ratios Statutory Accounting Practices Disclosure [Table Text Block] Investments, Debt and Equity Securities [Abstract] Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] State licenses State Licenses [Member] State Licenses [Member] GSE applications GSE Approvals [Member] GSE Approvals [Member] Indefinite-lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Goodwill Goodwill Intangible assets Indefinite-lived Intangible Assets (Excluding Goodwill) Total intangible assets and goodwill Impairment loss related to intangible assets or goodwill Goodwill and Intangible Asset Impairment Commitments and Contingencies Disclosure [Abstract] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Totals Operating Leases, Future Minimum Payments Due Equity [Abstract] Common Stock Offering Stockholders' Equity Note Disclosure [Text Block] Quarterly Financial Information Disclosure [Abstract] Net premiums earned Premiums Earned, Net Net investment income Net Investment Income Net realized investment gains (losses) Gain (Loss) on Investments Other revenues Other Income Insurance claims and claim expenses Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability Underwriting and operating expenses Other Cost and Expense, Operating Loss from change in fair value of warrant liability Gain (Loss) on Change in Fair Value of Warrant Liability Gain (Loss) on Change in Fair Value of Warrant Liability Interest expense Interest Expense Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Income tax expense (benefit) Net income Net Income (Loss) Attributable to Parent Income (loss) per share: (1) Earnings Per Share [Abstract] Basic earnings per share (in dollars per share) Earnings Per Share, Basic Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Basic weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Diluted Revenues Revenues [Abstract] Net realized investment gains Total revenues Revenues Expenses Costs and Expenses [Abstract] Other operating expenses Other Expenses Total expenses Costs and Expenses Other expense Other Income and Expenses [Abstract] Interest expense Total other expenses Other Nonoperating Income (Expense) Equity in net income of subsidiaries Income (Loss) from Equity Method Investments Income before income taxes Net income Other comprehensive (loss) income, net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized gains in accumulated other comprehensive loss, net of tax (benefit) expense of ($34), ($49), and $82 for each of the years in the three-year period ended December 31, 2018, respectively Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Reclassification adjustment for losses (gains) included in net loss, net of tax (benefit) expense of ($1), $0 and $0 for each of the years in the three-year period ended December 31, 2018 Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Equity in other comprehensive income (loss) of subsidiaries Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Subsidiary Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Subsidiary Other comprehensive (loss) income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Class A Common Class A [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] MAC Financial Holding Corporation and Subsidiaries MAC Financial Holding Corporation and Subsidiaries [Member] MAC Financial Holding Corporation and Subsidiaries [Member] Sale of Stock [Axis] Sale of Stock [Axis] Sale of Stock [Domain] Sale of Stock [Domain] IPO IPO [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Common stock offered and sold (in shares) Stock Issued During Period, Shares, New Issues Proceeds from issuance of common stock, net of stock issuance costs Proceeds From Issuance of Common Stock, Net of Stock Issuance Costs Proceeds From Issuance of Common Stock, Net of Stock Issuance Costs Cash, common stock and warrants issued for acquisition Business Combination, Consideration Transferred, Cash and Equity Interests Issued and Issuable Business Combination, Consideration Transferred, Cash and Equity Interests Issued and Issuable Liabilities assumed in acquisition Business Combination, Consideration Transferred, Liabilities Incurred Number of states in which entity operates Number of States in which Entity Operates Gross realized investment gains Debt Securities, Available-for-sale, Realized Gain Gross realized investment losses Debt Securities, Available-for-sale, Realized Loss Net realized investment gains (losses) Debt Securities, Available-for-sale, Realized Gain (Loss) Document and Entity Information [Abstract] Document and Entity Information [Abstract] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Public Float Entity Public Float Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Fair Value Disclosures [Abstract] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Valuation Technique, Option Pricing Model Valuation Technique, Option Pricing Model [Member] Measurement Input Type [Axis] Measurement Input Type [Axis] Measurement Input Type [Domain] Measurement Input Type [Domain] Common stock price Measurement Input, Share Price [Member] Risk free interest rate Measurement Input, Risk Free Interest Rate [Member] Expected life Measurement Input, Expected Term [Member] Expected volatility Measurement Input, Option Volatility [Member] Dividend yield Measurement Input, Expected Dividend Rate [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Warrant liability, measurement input Warrants and Rights Outstanding, Measurement Input Warrant liability, expected life (in years) Warrants and Rights Outstanding, Term Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Share-Based Compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Investment income Investment Income, Interest and Dividend Investment expenses Investment Income, Investment Expense Net investment income Investment Income, Net Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Geographic Concentration Risk Geographic Concentration Risk [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Risk-in-Force Risk In Force Claims [Member] Risk In Force Claims [Member] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Software Software and Software Development Costs [Member] Equipment Equipment [Member] ASU No. 2016-02 Accounting Standards Update 2016-02 [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Retained Earnings Retained Earnings [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Concentration risk Concentration Risk, Percentage IBNR default period (at least) Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Default Period Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Default Period Amortization of deferred acquisition expense Deferred Policy Acquisition Costs, Amortization Expense Premium deficiency reserve expense Premium Deficiency Reserve, Expense Premium Deficiency Reserve, Expense Estimated useful life Property, Plant and Equipment, Useful Life Mortgage insurance premiums receivable threshold for write off (in days) Threshold Period Past Due For Write-Off Of Mortgage Insurance Premiums Receivable Threshold period past due for mortgage insurance premium receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Right-of-use lease assets Operating Lease, Right-of-Use Asset Operating lease liabilities Operating Lease, Liability Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] U.S. Treasury securities and obligations of U.S. government agencies US Treasury and Government [Member] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Cash and investments held with various state insurance departments Cash, Cash Equivalents, and Short-term Investments Unrealized loss position, accumulated loss Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss Unrealized loss position, 12 months or greater Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Other-than-temporary impairment Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale Deferred tax asset Deferred Tax Assets, Gross [Abstract] Net operating loss carry forwards Deferred Tax Assets, Operating Loss Carryforwards Share-based compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Unearned premium reserve Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Unearned Premiums Reserve Accrued expenses Deferred Tax Assets, Deferred Ceding Commissions Deferred Tax Assets, Deferred Ceding Commissions Unrealized loss on investments Deferred Tax Assets, Tax Deferred Expense, Start-up Costs Deferred Tax Assets, Tax Deferred Expense, Start-up Costs Deferred ceding commissions Deferred Tax Assets, Investments Capitalized start-up costs Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax Other Deferred Tax Assets, Other Total gross deferred tax assets Deferred Tax Assets, Gross Less: valuation allowance Total deferred tax assets Deferred Tax Assets, Net of Valuation Allowance Deferred tax liability Deferred Tax Liabilities, Gross [Abstract] Contingency reserve Deferred Tax Liabilities, Deferred Expense, Reserves and Accruals Deferred acquisition costs Deferred Tax Liabilities, Deferred Expense, Deferred Policy Acquisition Cost Capitalized software Deferred Tax Liabilities, Deferred Expense, Capitalized Software Intangible assets Deferred Tax Liabilities, Investments Intangible assets Deferred Tax Liabilities, Intangible Assets Other Deferred Tax Liabilities, Other Total deferred tax liabilities Deferred Tax Liabilities, Gross Net deferred tax (liability)/assets Net deferred tax (liability)/assets Schedule of Quarterly Financial Information Quarterly Financial Information [Table Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 2018 Term Loan Senior Secured Term Loan 2018 [Member] Senior Secured Term Loan 2018 [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Secured Debt Secured Debt [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Unamortized issuance cost and original issue discount Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net Fair value of term loan Long-term Debt, Fair Value Short-duration Insurance Contracts, Historical Claims Duration [Table] Short-duration Insurance Contracts, Historical Claims Duration [Table] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Financial Guarantee Insurance Product Line Financial Guarantee Insurance Product Line [Member] Average annual percentage payout of incurred claims and allocated claims adjustment expenses by age, net of reinsurance Short-duration Insurance Contracts, Historical Claims Duration [Line Items] Year 1 Short-duration Insurance Contracts, Historical Claims Duration, Year One Year 2 Short-duration Insurance Contracts, Historical Claims Duration, Year Two Year 3 Short-duration Insurance Contracts, Historical Claims Duration, Year Three Year 4 Short-duration Insurance Contracts, Historical Claims Duration, Year Four Year 5 Short-duration Insurance Contracts, Historical Claims Duration, Year Five Debt Debt Disclosure [Text Block] Condensed Cash Flow Statement [Table] Condensed Cash Flow Statement [Table] Condensed Cash Flow Statements, Captions [Line Items] Condensed Cash Flow Statements, Captions [Line Items] Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Loss from change in fair value of warrant liability Fair Value Adjustment of Warrants Net realized investment (gains) Depreciation and amortization Depreciation, Amortization and Accretion, Net Amortization of debt discount and debt issuance costs Amortization of Debt Issuance Costs and Discounts Deferred income taxes Deferred Income Taxes and Tax Credits Share-based compensation expense Share-based Compensation Changes in operating assets and liabilities: Increase (Decrease) in Other Operating Assets and Liabilities, Net [Abstract] Equity in net (income) loss of subsidiaries (1) Gain (Loss) on Disposition of Stock in Subsidiary or Equity Method Investee Accrued investment income Increase (Decrease) in Accrued Investment Income Receivable Receivable from affiliates Increase (Decrease) Due from Affiliates Prepaid expenses Increase (Decrease) in Prepaid Expense Other assets Increase (Decrease) in Prepaid Expense and Other Assets Accounts payable and accrued expenses Increase (Decrease) in Accounts Payable and Accrued Liabilities Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Capitalization of subsidiaries Payments to Acquire Interest in Subsidiaries and Affiliates Purchase of short-term investments Payments to Acquire Short-term Investments Purchase of fixed-maturity investments, available-for-sale Payments to Acquire Debt Securities, Available-for-sale Proceeds from maturity of short-term investments Proceeds from Sale of Short-term Investments Proceeds from redemptions, maturities and sale of fixed-maturity investments, available-for-sale Proceeds from Sale and Maturity of Debt Securities, Available-for-sale Software and equipment Payments to Acquire Property, Plant, and Equipment Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of common stock related to public offering, net of issuance costs Proceeds from Issuance of Common Stock Proceeds from issuance of common stock related to employee equity plans Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options Proceeds from issuance of common stock related to warrants Proceeds from Warrant Exercises Taxes paid related to net share settlement of equity awards Payments Related to Tax Withholding for Share-based Compensation Proceeds from term loan Proceeds from Issuance of Long-term Debt Repayments of term loan Repayments of Long-term Debt Payments of debt issuance/modification costs Payments of Debt Issuance Costs Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Net increase (decrease) in cash, cash equivalents and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash, cash equivalents and restricted cash, beginning of period Cash, cash equivalents and restricted cash, end of period Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs (Level 2) Municipal debt securities US States and Political Subdivisions Debt Securities [Member] Corporate debt securities Corporate Debt Securities [Member] Asset-backed securities Asset-backed Securities [Member] Long-term investments - other Other Debt Obligations [Member] Fair value of securities Cash, cash equivalents and short-term investments Cash and Cash Equivalents, Fair Value Disclosure Total assets Assets, Fair Value Disclosure Warrant liability Warrant Liability Warrant Liability Total liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Regulatory Information Insurance Disclosure [Text Block] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Statutory Accounting Practices [Table] Statutory Accounting Practices [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] NMIC NMIC [Member] NMIC [Member] Subsidiaries Subsidiaries [Member] Statutory Accounting Practices [Line Items] Statutory Accounting Practices [Line Items] Maximum permitted RTC ratio Maximum Permitted RTC Ratio Maximum Permitted RTC Ratio Number of states which require minimum amount of statutory capital relative to risk in force Number of States Which Require Minimum Amount of Statutory Capital Relative to Risk In Force Number of States Which Require Minimum Amount of Statutory Capital Relative to Risk In Force Mortgage insurance risk in force Mortgage Insurance Risk in Force This concept represents the risk in force (an estimate of the maximum loss exposure that an entity has at any point in time). Risk-to-Capital Risk To Capital Ratio Regulatory Actual This concept represents an entity's ratio of net risk in force (excludes risk ceded through reinsurance contracts and risk in force on defaulted loans) to the amount of statutory capital (defined as the sum of statutory policyholders' surplus plus statutory contingency reserves). Mortgage insurance, percentage of indebtedness on single loan Mortgage Insurance, Percentage of Indebtedness on Single Loan, Limit Mortgage Insurance, Percentage of Indebtedness on Single Loan, Limit Shareholders' equity Dividends, restriction with regards to capital surplus Ordinary Dividends, Restriction with Regards to Capital Surplus This concept represents the maximum percentage of surplus as of the prior year that an insurer may pay out in ordinary dividends without prior approval of the Wisconsin Office of the Commissioner of Insurance under Wisconsin law. Dividends, restriction with regards to capital surplus Ordinary Dividends, Restriction with Regards to Capital Surplus, Next Fiscal Year Ordinary Dividends, Restriction with Regards to Capital Surplus, Next Fiscal Year Investment in subsidiaries, at equity in net assets Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Reconciliation of Liability for Insurance Claims and Claims Expenses Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Schedule of Claims Development Data Short-duration Insurance Contracts, Claims Development [Table Text Block] Schedule of Reconciliation of Claims Development to Liability Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table Text Block] Schedule of Historical Claims Duration Short-duration Insurance Contracts, Schedule of Historical Claims Duration [Table Text Block] Statement of Stockholders' Equity [Abstract] Common Stock - Class A Common Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Retained Earnings (Accumulated Deficit) Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning balance Beginning balance (in shares) Shares, Outstanding Cumulative effect of change in accounting principle Common stock: class A shares issued related to public offering Stock Issued During Period, Value, New Issues Common stock: class A shares issued related to public offering (in shares) Common stock: class A shares issued related to warrants Stock Issued During Period, Value, Conversion of Convertible Securities Common stock: class A shares issued related to warrants (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes Stock Issued During Period, Value, Employee Stock Purchase Plan, Net of Tax Withholdings Stock Issued During Period, Value, Employee Stock Purchase Plan, Net of Tax Withholdings Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans, Net of Shares For Tax Withholdings Stock Issued During Period, Shares, Employee Stock Purchase Plans, Net of Shares For Tax Withholdings Share-based compensation expense Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Change in unrealized investment gains/losses, net of tax ($3,258 benefit in 2018, $1,307 expense in 2017; $1,178 expense in 2016) Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax Net income Ending balance Ending balance (in shares) Condensed Financial Statements [Table] Condensed Financial Statements [Table] Condensed Financial Statements, Captions [Line Items] Condensed Financial Statements, Captions [Line Items] Operating underwriting and operating expenses Operating Expenses Income Taxes Income Tax Disclosure [Text Block] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Beginning balance Less reinsurance recoverables Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments Beginning balance, net of reinsurance recoverables Liability for Unpaid Claims and Claims Adjustment Expense, Net Claims and claim expenses incurred: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Current year Current Year Claims and Claims Adjustment Expense Prior years Prior Year Claims and Claims Adjustment Expense Total claims and claims expenses incurred Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Claims and claim expenses paid: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total claims and claim expenses paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Reserve at end of period, net of reinsurance recoverables Add reinsurance recoverables Ending balance Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Liability Class [Axis] Liability Class [Axis] Fair Value by Liability Class [Domain] Fair Value by Liability Class [Domain] Warrant Liability Warrant Liability [Member] Warrant Liability [Member] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Change in fair value of warrant liability included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Issuance of common stock on warrant exercise Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Ending balance Quarterly Financial Data (Unaudited) Quarterly Financial Information [Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Restricted Stock Units, Shares [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Non-vested restricted stock units, beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Restricted stock units granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Restricted stock units vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Restricted stock units forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Non-vested restricted stock units, ending balance (in shares) Restricted Stock Units, Weighted Average Grant Date Fair Value [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Non-vested restricted stock units, beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Restricted Stock Units Granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Restricted Stock Units Vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Restricted Stock Units Forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Non-vested restricted stock units, ending balance (in dollars per share) Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statements, Captions [Line Items] Condensed Balance Sheet Statements, Captions [Line Items] Cash and cash equivalents Due from affiliates, net Due from Affiliates Deferred tax liability, net Equity [Abstract] Schedule of Future Principal Payments Schedule of Maturities of Long-term Debt [Table Text Block] Income Statement [Abstract] Revenues Other expense Other Income, Nonoperating [Abstract] Total other expense Earnings per share (EPS) Earnings Per Share, Basic and Diluted [Abstract] Weighted average common shares outstanding Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Other comprehensive (loss) income, net of tax: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized (losses) gains in accumulated other comprehensive income, net of tax (benefit) expense of ($3,285), $1,234 and $1,178 for each of the years in the three-year period ended December 31, 2018, respectively Reclassification adjustment for realized losses (gains) included in net income, net of tax expense (benefit) of ($27), $73, and $0 for each of the years in the three-years ended December 31, 2018, respectively SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Abstract] Summary of Investments - Other than Investments in Related Parties SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Text Block] Change in unrealized investment gains/losses, tax expense (benefit) Other Comprehensive Income (Loss), Securities, Available-for-sale, Tax SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract] Financial Information of Registrant Reinsurance Parent Company Only SEC Schedule, 12-17, Insurance Companies, Reinsurance [Text Block] Short-duration Insurance Contracts, Claims Development [Table] Short-duration Insurance Contracts, Claims Development [Table] Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Domain] Short-duration Insurance Contracts, Accident Year [Domain] 2013 Short-duration Insurance Contracts, Accident Year 2013 [Member] 2014 Short-duration Insurance Contracts, Accident Year 2014 [Member] 2015 Short-duration Insurance Contracts, Accident Year 2015 [Member] 2016 Short-duration Insurance Contracts, Accident Year 2016 [Member] 2017 Short-duration Insurance Contracts, Accident Year 2017 [Member] 2018 Short-duration Insurance Contracts, Accident Year 2018 [Member] Claims Development [Line Items] Claims Development [Line Items] Cumulative Incurred Claims and Allocated Claims Adjustment Expenses, net of Reinsurance Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net Incurred but not reported claims liability Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net Cumulative number of reported claims Short-duration Insurance Contract, Cumulative Number of Reported Claims Cumulative Paid Claims and Allocated Claims Adjustment Expenses, net of Reinsurance Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net Stock Options Employee Stock Option [Member] Expected life (in years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Risk free interest rate, minimum (percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum Risk free interest rate, maximum (percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum Dividend yield (percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected stock price volatility, minimum (percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Minimum Expected stock price volatility, maximum (percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate, Maximum SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Table] SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Table] Total bonds Bonds [Member] Short-term investments Short-term Investments [Member] SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] Amortized Cost SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Cost Fair Value SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Fair Value Amount Reflected on Balance Sheet SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Amount Warrants issued (in shares) Class of Warrant or Right, Number of Securities Called by Warrants or Rights Right to purchase, number of shares per warrant (in shares) Number Of Shares, Right To Purchase Per Warrant Number Of Shares, Right To Purchase Per Warrant Exercise price of warrants (in dollars per warrant) Class of Warrant or Right, Exercise Price of Warrants or Rights Warrants exercised (in shares) Class of Warrant or Right, Number of Warrants Exercised During Period Class of Warrant or Right, Number of Warrants Exercised During Period Stock issued upon exercise of warrants (in shares) Fair value of warrant liability reclassified to additional paid in capital upon exercise Class Of Warrant Or Right, Fair Value Of Warrant Liability Reclassified To Additional Paid In Capital Upon Exercise Class Of Warrant Or Right, Fair Value Of Warrant Liability Reclassified To Additional Paid In Capital Upon Exercise Change in fair value of warrant liability in current period Class Of Warrant Or Right, Change In Fair Value Of Warrant Liability In Current Period Class Of Warrant Or Right, Change In Fair Value Of Warrant Liability In Current Period Fixed maturities, available-for-sale, amortized cost Debt Securities, Available-for-sale, Amortized Cost Restricted cash Restricted Cash Schedule of Components of Income Tax Expense (Benefit) Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Schedule of Deferred Tax Assets and Liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Statement of Cash Flows [Abstract] Net realized investment (gains) losses Net amortization of premium on investment securities Accretion (Amortization) of Discounts and Premiums, Investments Premiums receivable Increase (Decrease) in Premiums Receivable Deferred policy acquisition costs, net Increase (Decrease) in Deferred Policy Acquisition Costs Unearned premiums Increase (Decrease) in Unearned Premiums Reserve for insurance claims and claim expenses Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Reinsurance balances, net Increase (Decrease) in Reinsurance Assets and Liabilities Increase (Decrease) in Reinsurance Assets and Liabilities Supplemental disclosures of cash flow information Supplemental Cash Flow Information [Abstract] Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes paid Income Taxes Paid Future Principal Payments [Abstract] Long-term Debt, Fiscal Year Maturity [Abstract] 2019 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2020 Long-term Debt, Maturities, Repayments of Principal in Year Two 2021 Long-term Debt, Maturities, Repayments of Principal in Year Three 2022 Long-term Debt, Maturities, Repayments of Principal in Year Four 2023 Long-term Debt, Maturities, Repayments of Principal in Year Five Total Long-term Debt, Gross Stock Options, Outstanding [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Options beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Options granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Options exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Options forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Options expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Expirations in Period Options ending balance (in shares) Stock Options, Weighted Average Grant Date Fair Value Per Share [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Grant Date Fair Value [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Grant Date Fair Value [Roll Forward] Options beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Options granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Options exercised (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Weighted Average Grant Date Fair Value Options forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value Options expired (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Expired, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Expired, Weighted Average Grant Date Fair Value Options ending balance (in dollars per share) Stock Options, Weighted Average Exercise Price [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Options beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Options granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Options exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Options forfeited (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Options expired (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price Options beginning balance (in dollars per share) Ceded Credit Risk [Table] Ceded Credit Risk [Table] Ceded Credit Risk, Reinsurer [Axis] Ceded Credit Risk, Reinsurer [Axis] Ceded Credit Risk, Reinsurer [Domain] Ceded Credit Risk, Reinsurer [Domain] Third-Party Reinsurers Third-Party Reinsurers [Member] Third-Party Reinsurers [Member] Ceded Credit Risk [Line Items] Ceded Credit Risk [Line Items] Ceded risk-in-force Risk In Force, Ceded Risk In Force, Ceded Ceded premiums written Ceded Premiums Written Ceded premiums earned Ceded Premiums Earned Ceded claims and claims expenses Reinsurance Ceded Claims and Claims Expenses Reinsurance Ceded Claims and Claims Expenses Ceding commission written Reinsurance Ceding Commission Written Reinsurance Ceding Commission Written Ceding commission earned Reinsurance Ceding Commission Earned Reinsurance Ceding Commission Earned Profit commission Reinsurance Profit Commission Reinsurance Profit Commission Number of securities, less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions Fair value, less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Unrealized losses, less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Number of securities, 12 months or greater Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions Fair value, 12 months or greater Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer Unrealized losses, 12 months or greater Number of securities, total Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions Fair value Debt Securities, Available-for-sale, Unrealized Loss Position Unrealized losses Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Reinsurance Policy, Type [Axis] Reinsurance Policy, Type [Axis] Reinsurance Policy [Domain] Reinsurance Policy [Domain] 2016 QSR Transaction Two Thousand Sixteen QSR Transaction [Member] Two Thousand Sixteen QSR Transaction [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Fannie Mae Fannie Mae [Member] Fannie Mae [Member] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Primary loans in default Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Number of Primary Loans in Default Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Number of Primary Loans in Default Number of claims paid Liability for Paid Claims and Claims Adjustment Expense, Number of Paid Claims Liability for Paid Claims and Claims Adjustment Expense, Number of Paid Claims Claims paid, including amounts covered by insurance Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Including Amounts Covered By Insurance Amount of payments to settle insured claims and pay costs incurred in the claims settlement process, including amount covered by insurance. Number of covered claims included in number of claims paid Liability for Paid Claims and Claims Adjustment Expense, Number of Covered Claims Included In Number of Paid Claims Liability for Paid Claims and Claims Adjustment Expense, Number of Covered Claims Included In Number of Paid Claims Component of claims paid covered under QSR Transaction Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Including Amounts Covered By Insurance, Amount Under QSR Transaction Component of total amount of payments to settle insured claims and pay costs incurred in the claims settlement process, including amount covered by insurance, covered under QSR transaction. Number of loans in pool past due 60 days or more Loans In Pool, Past Due 60 Days Or More, Number Of Loans Loans In Pool, Past Due 60 Days Or More, Number Of Loans Risk-in-Force of loans in pool past due 60 days or more Loans In Pool, Past Due 60 Days Or More, Loan Risk in Force Loans In Pool, Past Due 60 Days Or More, Risk in Force Loan-to-value ratio (less than) Loans In Pool, Loan To Value Ratio Loans In Pool, Loan To Value Ratio Claims applied to pool deductible Liability for Unpaid Claims and Claims Adjustment Expense, Claims Applied to Deductible Liability for Unpaid Claims and Claims Adjustment Expense, Claims Applied to Deductible Deductible on policy Mortgage Pool Insurance, Claim Deductible Mortgage Pool Insurance, Claim Deductible Percent of pool RIF reinsured Reinsurance Retention Policy, Reinsured Risk, Percentage Favorable prior year development Reserve for prior year insurance claims and claims expense Liability for Prior Year Claims and Claims Adjustment Expense Liability for Prior Year Claims and Claims Adjustment Expense Earnings per Share (EPS) Earnings Per Share [Text Block] NMIC and Re One Combined NMic and Re one Combined [Member] NMic and Re one Combined [Member] Statutory net loss Statutory Accounting Practices, Statutory Net Income Amount Statutory surplus Statutory Accounting Practices, Statutory Capital and Surplus, Balance Contingency reserve Statutory Accounting Practices, Contingency Reserve Contingency reserve recorded under Statutory Accounting Principles is a liability to protect against loss during periods of extreme contraction. Schedule of Stock Option Activity Share-based Compensation, Stock Options, Activity [Table Text Block] Schedule of Stock Options Valuation Assumptions Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Schedule of Restricted Stock Units Activity Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] Debt Securities By Industry [Axis] Debt Securities By Industry [Axis] Debt Securities By Industry [Axis] Debt Securities By Industry [Domain] Debt Securities By Industry [Domain] [Domain] for Debt Securities By Industry [Axis] Financial Financial, Industry Group [Member] Financial, Industry Group [Member] Consumer Consumer, Industry Group [Member] Consumer, Industry Group [Member] Communications Communications, Industry Group [Member] Communications, Industry Group [Member] Industrial Industrial, Industry Group [Member] Industrial, Industry Group [Member] Utilities Utilities, Industry Group [Member] Utilities, Industry Group [Member] Technology Technology, Industry Group [Member] Technology, Industry Group [Member] Energy Energy, Industry Group [Member] Energy, Industry Group [Member] Other Other, Industry Group [Member] Other, Industry Group [Member] Debt Securities By Industry [Line Items] Debt Securities By Industry [Line Items] Debt Securities By Industry [Line Items] Corporate debt securities as component of total (percent) Debt Securities, Corporate, Component Of Total. Percentage Debt Securities, Corporate, Component Of Total. Percentage Schedule of Fair Value Measurements of Financial Instruments Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] Roll-Forward of Level 3 Liabilities Measured at Fair Value Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Schedule of Key Inputs and Assumptions in Option-Pricing Model Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Basic EPS Net Income (Loss) Available to Common Stockholders, Operations, Basic [Abstract] Diluted EPS Net Income (Loss) Available to Common Stockholders, Operations, Diluted [Abstract] Warrant gain, net of tax Amount Of Dilutive Securities Warrants Amount of increase (decrease) to net income used for calculating diluted earnings per share (EPS), resulting from warrant losses (gains). Diluted net income Net Income (Loss) Attributable to Parent, Diluted Dilutive effect of issuable shares (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Diluted weighted average shares outstanding (in shares) Antidilutive securities excluded from EPS calculation (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Net premiums written Premiums Written, Net [Abstract] Direct Direct Premiums Written Ceded Net premiums written Premiums Written, Net Net premiums earned Premiums Earned, Net [Abstract] Direct Direct Premiums Earned Ceded Net premiums earned Federal statutory income tax rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent State Provision Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Share-based and other compensation Effective Income Tax Rate Reconciliation, Compensation, Share-Based And Other, Percent Effective Income Tax Rate Reconciliation, Compensation, Share-Based And Other, Percent Warrant gain/loss Effective Income Tax Rate Reconciliation, Warrant Gain (Loss), Percent Effective Income Tax Rate Reconciliation, Warrant Gain (Loss), Percent Other Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Re-measurement from change in federal statutory rate Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent Valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent True-up from prior year Effective Income Tax Rate Reconciliation, True-up From Prior Year, Percent Effective Income Tax Rate Reconciliation, True-up From Prior Year, Percent Effective income tax rate Effective Income Tax Rate Reconciliation, Percent Amortized cost Gross unrealized gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Gross unrealized losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net Reinsurance recoverable on unpaid claims Unallocated claims adjustment expenses Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense Total gross liability for unpaid claims and claim adjustment expenses Mortgage-linked Debt 2017 ILN Notes Two Thousand Seventeen ILN Notes [Member] Two Thousand Seventeen ILN Notes [Member] 2018 ILN Notes Two Thousand Eighteen ILN Notes [Member] Two Thousand Eighteen ILN Notes [Member] Oaktown Re II Oaktown Re Ltd [Member] Oaktown Re Ltd [Member] 2017 ILN Transaction Two Thousand Seventeen ILN Transaction [Member] Two Thousand Seventeen ILN Transaction [Member] 2018 ILN Transaction Two Thousand Eighteen ILN Transaction [Member] Two Thousand Eighteen ILN Transaction [Member] QSR Transactions QSR Transactions [Member] QSR Transactions [Member] 2018 QSR Transaction Two Thousand Eighteen QSR Transaction [Member] Two Thousand Eighteen QSR Transaction [Member] Risk Written Policies in 2018 Eligible Policies, Pool Four [Member] Eligible Policies, Pool Four [Member] Risk Written Policies in 2019 Eligible Policies, Pool Five [Member] Eligible Policies, Pool Five [Member] Existing Risk Written Policies Eligible Policies, Pool One [Member] Eligible Policies, Pool One [Member] Fannie Mae Eligible Policies, Pool Two [Member] Eligible Policies, Pool Two [Member] Risk Written Policies from September 1, 2016 through December 31, 2017 Eligible Policies, Pool Three [Member] Eligible Policies, Pool Three [Member] Aggregate excess of loss reinsurance coverage Reinsurance Retention Policy, Excess Retention, Amount Reinsured Aggregate excess of loss reinsurance retained by company Reinsurance Retention Policy, Amount Retained Reinsurance coverage, term of underlying mortgage amortization (in years) Reinsurance Retention Policy, Term Of Underlying Mortgage Amortization Reinsurance Retention Policy, Term Of Underlying Mortgage Amortization Proceeds from issuance of notes Proceeds from Issuance of Debt Anticipated payment related to annual operating expenses Anticipated Operating Expense Payments, Annual Amount of anticipated payments related to operating expenses of counterparty. Risk premiums paid Reinsurance Retention, Risk Premiums Paid Reinsurance Retention, Risk Premiums Paid Optional termination right, percent of reinsurance coverage threshold Reinsurance Agreement, Optional Termination Right, Percent Of Reinsurance Coverage Threshold Reinsurance Agreement, Optional Termination Right, Percent Of Reinsurance Coverage Threshold Percent of premiums ceded under QSR Transaction Premiums of premiums earned under QSR Transaction Premiums Earned, Percent Of Risk Premiums Earned, Percent Of Risk Ceding commissions under QSR Transaction Ceded Credit Risk, Commission Earned, Percentage Ceded Credit Risk, Commission Earned, Percentage Threshold for loss ratio on loans to qualify for profit commission Ceded Credit Risk, Profit Commission, Loss Ratio On Loans Threshold Ceded Credit Risk, Profit Commission, Loss Ratio On Loans Threshold Schedule of Fair Values and Gross Unrealized Gains and Losses Debt Securities, Available-for-sale [Table Text Block] Schedule of Investments by Industry Group Investments Classified By Issuer Industry Group [Table Text Block] Investments Classified By Issuer Industry Group [Table Text Block] Schedule of Investments by Maturity Investments Classified by Contractual Maturity Date [Table Text Block] Schedule of Aging Unrealized Losses Schedule of Unrealized Loss on Investments [Table Text Block] Schedule of Net Investment Income Investment Income [Table Text Block] Schedule of Net Realized Investment Gains (Losses) Realized Gain (Loss) on Investments [Table Text Block] Approved insurers, required assets (equal or greater than) Approved Insurers, Minimum Required Assets Approved Insurers, Minimum Required Assets Approved insurers, risked-based required assets, primary insurance floor Approved Insurers, Risked-Based Required Assets, Primary Insurance Floor Approved Insurers, Risked-Based Required Assets, Primary Insurance Floor Rent expense Operating Leases, Rent Expense, Net Schedule of Effects of Reinsurance Agreements Effects of Reinsurance [Table Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility Revolving Credit Facility [Member] Senior Secured Debt Senior Secured Debt [Member] Senior Secured Debt [Member] 2018 Revolving Credit Facility Secured Revolving Credit Facility 2018 [Member] Secured Revolving Credit Facility 2018 [Member] 2015 Term Loan Credit Agreement Amendment [Member] Credit Agreement Amendment [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Base Rate Base Rate [Member] Eurodollar Eurodollar [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Debt instrument face amount Debt Instrument, Face Amount Debt instrument term (years) Debt Instrument, Term Credit facility borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Variable rate floor (percent) Debt Instrument, Variable Interest Rate Floor Debt Instrument, Variable Interest Rate Floor Basis spread on variable rate (percent) Debt Instrument, Basis Spread on Variable Rate Interest rate during period (percent) Debt Instrument, Interest Rate During Period Frequency of periodic payment Debt Instrument, Frequency of Periodic Payment Periodic payment of principal Debt Instrument, Periodic Payment, Principal Prepayment premium (percent) Debt Instrument, Prepayment Penalty Debt Instrument, Prepayment Penalty Prepayment penalty time period (months) Debt Instrument, Prepayment Penalty, Period Debt Instrument, Prepayment Penalty, Period Interest expense Cost related to extinguishment of debt Gain (Loss) on Extinguishment of Debt Deferred debt issuance costs Debt Issuance Costs, Gross Deferred debt issuance costs attributable to modified lenders Deferred Finance Costs, Attributable To Modified Lender, Gross Amount, before accumulated amortization, of debt issuance costs that are attributable to modified lenders. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. Original issue discount Debt Instrument, Unamortized Discount Original issue discount attributable to modified lenders Debt Instrument, Unamortized Discount Attributable To Modified Lenders Amount, after accumulated amortization, of debt discount that is attributable To modified lenders. Term loan Debt instrument covenant, maximum debt-to-total capitalization ratio Debt Instrument, Covenant, Debt to Total Capitalization Ratio, Maximum Debt Instrument, Covenant, Debt to Total Capitalization Ratio, Maximum Borrowings outstanding Line of Credit Facility, Fair Value of Amount Outstanding Commitment fee (percent) Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Commitment fees in interest expense Line of Credit Facility, Commitment Fee Amount Amortization of deferred issuance costs Amortization of Debt Issuance Costs Remaining deferred issuance costs, net of accumulated amortization Debt Issuance Costs, Line of Credit Arrangements, Net Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 2012 Stock Incentive Plan The 2012 Stock Incentive Plan [Member] The 2012 Stock Incentive Plan [Member] 2014 Omnibus Incentive Plan NMIH 2014 Omnibus Incentive Plan [Member] NMIH 2014 Omnibus Incentive Plan [Member] RSUs Subject to Service Conditions Restricted Stock Units (RSUs), Subject to Service Conditions [Member] Restricted Stock Units (RSUs), Subject to Service Conditions [Member] RSUs Subject to Service and Performance Conditions Restricted Stock Units (RSUs), Subject to Service And Performance Conditions [Member] Restricted Stock Units (RSUs), Subject to Service And Performance Conditions [Member] Number of shares authorized to be reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Additional shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Expiration period (not more than) Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Award requisite service period Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Income tax benefit related to share-based compensation Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Number of options vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number Options exercised (in shares) Aggregate intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Weighted average exercise price for options vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price Weighted average remaining contractual term Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term Options vested aggregate value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value Unrecognized compensation cost related to non-vested stock options Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options Unrecognized compensation cost, period for recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Restricted stock units outstanding (in shares) Share-based awards vesting period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Fair value of shares vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Weighted average remaining contractual life of RSUs outstanding Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Unrecognized compensation cost related to RSUs Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options Employer matching contribution, percent of match (up to) Defined Contribution Plan, Employer Matching Contribution, Percent of Match Employer matching contribution, percent of employees' gross pay (up to) Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Contribution amount Defined Contribution Plan, Employer Discretionary Contribution Amount Share-based awards granted (in shares) Schedule of Future Minimum Lease Payments Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Overallotment Option Over-Allotment Option [Member] Class of Stock [Line Items] Class of Stock [Line Items] Common stock sold excluding overallotment shares (in shares) Stock Issued During Period, Shares, New Issues Before Overallotment Stock Issued During Period, Shares, New Issues Before Overallotment Overallotment option included in sale of stock (percent) Stock Issued During Period, Overallotment Option, Percent Stock Issued During Period, Overallotment Option, Percent Proceeds from issuance of common stock Current Current Income Tax Expense (Benefit) Deferred Deferred Income Tax Expense (Benefit) Total income tax expense (benefit) Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Leasehold improvements Leasehold Improvements [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Property plant and equipment Property, Plant and Equipment, Gross Accumulated amortization and depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Software and equipment, net Intangible Assets and Goodwill Intangible Assets Disclosure [Text Block] SEC Schedule, 12-17, Insurance Companies, Reinsurance [Table] SEC Schedule, 12-17, Insurance Companies, Reinsurance [Table] SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] Gross Amount Ceded to Other Companies Assumed from Other Companies Assumed Premiums Earned Net Amount Percentage of Amount Assumed to Net SEC Schedule, 12-17, Insurance Companies, Reinsurance, Premium, Percentage Assumed to Net Amortized Cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost Due after one through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost Due after five through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost Due after ten years Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost Asset-backed securities Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Fair Value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Due after one through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Due after five through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Due after ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Asset-backed securities Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Fair value Organization and Basis of Presentation Business Description and Basis of Presentation [Text Block]