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Cover
Oct. 23, 2025
Document Type 8-K
Amendment Flag false
Document Period End Date Oct. 23, 2025
Entity File Number 814-00832
Entity Registrant Name New Mountain Finance Corporation
Entity Central Index Key 0001496099
Entity Tax Identification Number 27-2978010
Entity Incorporation, State or Country Code DE
Entity Address, Address Line One 1633 Broadway
Entity Address, Address Line Two 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
Common stock, par value $0.01 per share  
Title of 12(b) Security Common stock, par value $0.01 per share
Trading Symbol NMFC
Security Exchange Name NASDAQ
8.250% Notes due 2028  
Title of 12(b) Security 8.250% Notes due 2028
Trading Symbol NMFCZ
Security Exchange Name NASDAQ