Fair Value - Unobservable Inputs (Details)
$ in Thousands
Sep. 30, 2023
USD ($)
Dec. 31, 2022
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 3,111,903 [1] $ 3,221,247 [2]
First lien    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 1,691,480 1,753,967
Second lien    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 529,613 561,207
Subordinated    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 86,238 76,659
Equity and other    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 804,572 829,414
Level III    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 3,027,649 3,136,291
Level III | First lien    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 1,680,667 1,753,967
Level III | First lien | Market & income approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 1,652,759 $ 1,663,116
Level III | First lien | Market & income approach | EBITDA multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 1.3 4.8
Level III | First lien | Market & income approach | EBITDA multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 32.5 38.0
Level III | First lien | Market & income approach | EBITDA multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 15.2 15.3
Level III | First lien | Market & income approach | Revenue multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 4.0 5.0
Level III | First lien | Market & income approach | Revenue multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 19.5 19.5
Level III | First lien | Market & income approach | Revenue multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 7.0 9.5
Level III | First lien | Market & income approach | Discount rate | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.094 0.083
Level III | First lien | Market & income approach | Discount rate | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.254 0.294
Level III | First lien | Market & income approach | Discount rate | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.116 0.111
Level III | First lien | Other    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 27,908 $ 90,851
Level III | Second lien    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 461,833 480,068
Level III | Second lien | Market & income approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 458,833 $ 471,350
Level III | Second lien | Market & income approach | EBITDA multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 7.8 8.2
Level III | Second lien | Market & income approach | EBITDA multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 21.0 32.0
Level III | Second lien | Market & income approach | EBITDA multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 15.0 15.2
Level III | Second lien | Market & income approach | Discount rate | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.103 0.112
Level III | Second lien | Market & income approach | Discount rate | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.340 0.471
Level III | Second lien | Market & income approach | Discount rate | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.125 0.137
Level III | Second lien | Other    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 3,000 $ 8,718
Level III | Subordinated    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value 80,577 72,842
Level III | Subordinated | Market & income approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 80,577 $ 72,842
Level III | Subordinated | Market & income approach | EBITDA multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 9.0 8.0
Level III | Subordinated | Market & income approach | EBITDA multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 32.5 23.5
Level III | Subordinated | Market & income approach | EBITDA multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 16.2 16.3
Level III | Subordinated | Market & income approach | Discount rate | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.130 0.135
Level III | Subordinated | Market & income approach | Discount rate | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.229 0.297
Level III | Subordinated | Market & income approach | Discount rate | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.165 0.170
Level III | Equity and other    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 804,572 $ 829,414
Level III | Equity and other | Market & income approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 798,186 $ 793,468
Level III | Equity and other | Market & income approach | EBITDA multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 5.5 4.8
Level III | Equity and other | Market & income approach | EBITDA multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 26.5 26.5
Level III | Equity and other | Market & income approach | EBITDA multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 12.5 13.0
Level III | Equity and other | Market & income approach | Revenue multiple | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 4.0 10.5
Level III | Equity and other | Market & income approach | Revenue multiple | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 20.1 19.5
Level III | Equity and other | Market & income approach | Revenue multiple | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 7.5 14.6
Level III | Equity and other | Market & income approach | Discount rate | Low    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.064 0.064
Level III | Equity and other | Market & income approach | Discount rate | High    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.440 0.440
Level III | Equity and other | Market & income approach | Discount rate | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Measurement Input 0.116 0.130
Level III | Equity and other | Other    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value $ 6,386 $ 35,946
[1] New Mountain Finance Corporation (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act
[2] New Mountain Finance Corporation (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act.