| Cover | Feb. 26, 2024 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 26, 2024 |
| Entity Registrant Name | New Mountain Finance Corporation |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 814-00832 |
| Entity Tax Identification Number | 27-2978010 |
| Entity Address, Address Line One | 1633 Broadway, 48th Floor |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10019 |
| City Area Code | 212 |
| Local Phone Number | 720-0300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001496099 |
| Amendment Flag | false |
| Common Stock | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | NMFC |
| Security Exchange Name | NASDAQ |
| 8.250% Unsecured Notes Due 2028 | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 8.250% Notes due 2028 |
| Trading Symbol | NMFCZ |
| Security Exchange Name | NASDAQ |
