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Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Feb. 27, 2023
Jun. 30, 2022
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2022    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 814-00832    
Entity Registrant Name New Mountain Finance Corporation    
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    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 27-2978010    
Title of 12(b) Security Common stock, par value $0.01 per share    
Trading Symbol NMFC    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 1,077.8
Entity Common Stock, Shares Outstanding   100,937,026  
Documents Incorporated by Reference Portions of the Registrant's Proxy Statement for its 2023 Annual Meeting of Stockholders to be filed not later than 120 days after the end of the fiscal year covered by this Annual Report on this Form 10-K are incorporated by reference into Part III on this Form 10-K.    
Entity Central Index Key 0001496099    
Amendment Flag false    
Document Fiscal Year Focus 2022    
Document Fiscal Period Focus FY