Fair Value - Unobservable Inputs (Details) $ in Thousands	Sep. 30, 2024 USD ($)		Dec. 31, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 3,265,271	[1]	$ 3,011,322	[2]
Level III
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	3,141,885		2,938,849
Level III | First lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,796,760		$ 1,559,300
Level III | First lien | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	5.0		5.0
Level III | First lien | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	35.0		24.0
Level III | First lien | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15.1		15.1
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	8.0		10.6
Level III | First lien | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.065		0.086
Level III | First lien | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.211		0.220
Level III | First lien | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.097		0.102
Level III | First lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 222,303		$ 78,589
Level III | Second lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 175,751		$ 403,180
Level III | Second lien | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.6		7.0
Level III | Second lien | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	21.0		20.0
Level III | Second lien | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	18.2		14.4
Level III | Second lien | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.095		0.092
Level III | Second lien | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.210		0.300
Level III | Second lien | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.114		0.121
Level III | Second lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 15,581		$ 3,000
Level III | Subordinated | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 99,386		$ 82,871
Level III | Subordinated | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.7		8.0
Level III | Subordinated | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	24.5		22.0
Level III | Subordinated | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	16.3		16.0
Level III | Subordinated | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.122		0.129
Level III | Subordinated | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.238		0.209
Level III | Subordinated | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.169		0.119
Level III | Equity and other | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 424,106		$ 430,828
Level III | Equity and other | Market & income approach | EBITDA multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	5.0		5.5
Level III | Equity and other | Market & income approach | EBITDA multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	32.0		34.0
Level III | Equity and other | Market & income approach | EBITDA multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.0		12.6
Level III | Equity and other | Market & income approach | Revenue multiple | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	4.5		9.0
Level III | Equity and other | Market & income approach | Revenue multiple | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	19.5		11.0
Level III | Equity and other | Market & income approach | Revenue multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.1		10.0
Level III | Equity and other | Market & income approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.084		0.098
Level III | Equity and other | Market & income approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.526		0.431
Level III | Equity and other | Market & income approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.092		0.122
Level III | Equity and other | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 10,718		$ 10,274
Level III | Equity and other | Income Approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 397,280		$ 370,807
Level III | Equity and other | Income Approach | Discount rate | Low
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.063		0.064
Level III | Equity and other | Income Approach | Discount rate | High
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.125		0.126
Level III | Equity and other | Income Approach | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.100		0.101

[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.