| Cover Page | Feb. 16, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 16, 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 |
