Cover Page
May 14, 2020
Cover [Abstract]  
Document Type 8-K
Document Period End Date May 13, 2021
Entity Registrant Name NMI Holdings, Inc.
Entity Central Index Key 0001547903
Amendment Flag false
Entity Incorporation, State or Country Code DE
Entity File Number 001-36174
Entity Tax Identification Number 45-4914248
Entity Address, Address Line One 2100 Powell Street
Entity Address, Address Line Two 12th Floor
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
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Title of 12(b) Security Class A Common Stock, par value $0.01
Trading Symbol NMIH
Security Exchange Name NASDAQ
Entity Emerging Growth Company false