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Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 26, 2024
Jun. 30, 2023
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
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    
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    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 1,120.1
Entity Common Stock, Shares Outstanding   102,877,604  
Documents Incorporated by Reference Portions of the Registrant's Proxy Statement for its 2024 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 2023    
Document Fiscal Period Focus FY    
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    
8.25% Notes due 2028      
Document Information [Line Items]      
Title of 12(b) Security 8.250% Notes due 2028    
Trading Symbol NMFCZ    
Security Exchange Name NASDAQ