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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.

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