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Investments - Selected Balance Sheet Information for SLP III and SLP IV (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Investments, cost | $ 3,038,208 | [1] | $ 3,259,967 | [2]
Investments at fair value | 3,011,322 | [1] | 3,221,247 | [2]
Total assets | 3,159,214 |  | 3,354,927 |
Deferred financing costs, accumulated amortization | 54,263 |  | 47,531 |
Deferred financing costs (net of accumulated amortization) | (22,387) |  | (17,199) |
Total liabilities | 1,827,491 |  | 2,028,736 |
Total liabilities and net assets | 3,159,214 |  | 3,354,927 |
SLP III |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Investments, cost | 646,315 |  | 681,288 |
Investments at fair value | 636,560 |  | 639,327 |
Cash and other assets | 21,443 |  | 17,149 |
Total assets | 658,003 |  | 656,476 |
Deferred financing costs, accumulated amortization | 5,650 |  | 4,840 |
Deferred financing costs (net of accumulated amortization) | (943) |  | (1,695) |
Payable for unsettled securities purchased | 23,881 |  | 0 |
Distribution payable | 6,672 |  | 5,688 |
Other liabilities | 7,862 |  | 6,492 |
Total liabilities | 490,672 |  | 522,585 |
Members' capital | 167,331 |  | 133,891 |
Total liabilities and net assets | 658,003 |  | 656,476 |
SLP III | Revolving Credit Facility |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Credit facility | 453,200 |  | 512,100 |
SLP IV |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Investments, cost | 476,017 |  | 504,875 |
Investments at fair value | 467,886 |  | 473,762 |
Cash and other assets | 16,227 |  | 12,853 |
Receivable from unsettled securities sold | 1,445 |  | 0 |
Total assets | 485,558 |  | 486,615 |
Deferred financing costs, accumulated amortization | 1,599 |  | 997 |
Deferred financing costs (net of accumulated amortization) | (1,414) |  | (2,008) |
Payable for unsettled securities purchased | 31,322 |  | 0 |
Distribution payable | 5,220 |  | 4,648 |
Other liabilities | 6,676 |  | 5,410 |
Total liabilities | 348,341 |  | 373,587 |
Members' capital | 137,217 |  | 113,028 |
Total liabilities and net assets | 485,558 |  | 486,615 |
SLP IV | Revolving Credit Facility |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Credit facility | $ 306,537 |  | $ 365,537 |
[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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