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Cover - shares
 9 Months Ended
Sep. 30, 2022
Nov. 08, 2022
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2022  
Document Transition Report false  
Entity File Number 814-00832  
Entity Registrant Name New Mountain Finance Corporation  
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  
Entity Incorporation, State or Country Code DE  
Title of 12(b) Security Common stock, par value $0.01 per share  
Trading Symbol NMFC  
Security Exchange Name NASDAQ  
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  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   100,937,026
Entity Central Index Key 0001496099  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2022  
Document Fiscal Period Focus Q3