v3.10.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Oct. 26, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name NMI HOLDINGS, INC.  
Entity Central Index Key 0001547903  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Amendment Flag false  
Entity Filer Category Accelerated Filer  
Entity Emerging Growth Company true  
Entity Small Business false  
Entity Ex Transition Period true  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Current Fiscal Year End Date --12-31  
Entity Common Stock, Shares Outstanding   66,302,317