XML 13 R1.htm IDEA: XBRL DOCUMENT v3.25.3
Cover - shares
 9 Months Ended
Sep. 30, 2025
Oct. 30, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2025  
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 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 (in shares)   76,867,928
Entity Central Index Key 0001547903  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q3  
Amendment Flag false