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COVER PAGE - shares
 6 Months Ended
Jun. 30, 2019
Jul. 29, 2019
Cover page.    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2019  
Document Transition Report false  
Entity File Number 001-36174  
Entity Registrant Name NMI Holdings, Inc.  
Entity Central Index Key 0001547903  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 45-4914248  
Entity Address, Address Line One 2100 Powell Street  
Entity Address, City or Town Emeryville  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94608  
City Area Code 855  
Local Phone Number 530-6642  
Title of 12(b) Security Class A Common Stock, par value $0.01  
Trading Symbol NMIH  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   67,768,466