Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 11, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 001-36174    
Entity Registrant Name NMI Holdings, Inc.    
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, $.01 par value per share    
Trading Symbol NMIH    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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 Public Float     $ 1,617,138,263
Entity Common Stock, Shares Outstanding   68,438,109  
Documents Incorporated by Reference the registrant's Proxy Statement for the 2020 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. Such Proxy Statement will be filed with the Securities and Exchange Commission within 120 days of the registrant's fiscal year ended December 31, 2019.    
Entity Central Index Key 0001547903    
Amendment Flag false    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --12-31