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Investments - Investment Cost and Fair Value by Type and by Industry (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule of Investments [Line Items] |  |  |  |
Cost | $ 3,168,719 | [1] | $ 3,259,967 | [2]
Fair Value | 3,111,903 | [1] | 3,221,247 | [2]
Software |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 831,652 |  | 916,259 |
Fair Value | 820,492 |  | 897,008 |
Business Services |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 606,262 |  | 661,079 |
Fair Value | 540,834 |  | 592,868 |
Healthcare |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 560,096 |  | 586,311 |
Fair Value | 535,665 |  | 548,383 |
Investment Funds (includes investments in joint ventures) |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 252,400 |  | 252,400 |
Fair Value | 252,400 |  | 252,400 |
Education |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 221,749 |  | 200,117 |
Fair Value | 249,225 |  | 239,301 |
Consumer Services |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 163,407 |  | 126,392 |
Fair Value | 161,335 |  | 123,880 |
Net Lease |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 95,782 |  | 96,041 |
Fair Value | 118,081 |  | 120,986 |
Distribution & Logistics |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 110,807 |  | 111,096 |
Fair Value | 104,712 |  | 102,410 |
Financial Services |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 102,058 |  | 96,021 |
Fair Value | 102,875 |  | 95,839 |
Energy |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 59,872 |  | 55,507 |
Fair Value | 63,935 |  | 61,564 |
Information Technology |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 58,688 |  | 58,638 |
Fair Value | 59,189 |  | 58,165 |
Packaging |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 42,964 |  | 43,124 |
Fair Value | 38,814 |  | 40,547 |
Specialty Chemicals & Materials |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 26,046 |  | 25,363 |
Fair Value | 27,804 |  | 60,268 |
Consumer Products |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 20,622 |  | 20,850 |
Fair Value | 19,014 |  | 20,624 |
Business Products |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 10,772 |  | 10,769 |
Fair Value | 10,788 |  | 7,004 |
Food & Beverage |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 5,542 |  |  |
Fair Value | 6,740 |  |  |
First lien |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 1,759,410 |  | 1,816,091 |
Fair Value | 1,691,480 |  | 1,753,967 |
Second lien |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 567,583 |  | 632,990 |
Fair Value | 529,613 |  | 561,207 |
Subordinated |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 95,993 |  | 85,774 |
Fair Value | 86,238 |  | 76,659 |
Equity and other |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Cost | 745,733 |  | 725,112 |
Fair Value | $ 804,572 |  | $ 829,414 |
[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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