v3.8.0.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2018
Apr. 27, 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 Mar. 31, 2018  
Amendment Flag false  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Common Stock, Shares Outstanding   65,573,093