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Investments - Selected Statement of Operations Information for SLP III and SLP IV (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Total investment income | $ 94,349 | $ 78,449 | $ 281,794 | $ 220,522
Interest and other financing expenses | 31,425 | 24,648 | 93,921 | 63,957
Total expenses | 55,315 | 46,768 | 165,575 | 129,693
Net investment income before income taxes | 40,047 | 32,796 | 119,391 | 94,416
Net realized losses on investments |  |  | 7,904 | 35,468
Net change in unrealized appreciation (depreciation) of investments |  |  | (18,340) | (67,634)
Net increase in net assets resulting from operations related to New Mountain Finance Corporation | 28,696 | 7,705 | 108,099 | 59,851
SLP III |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Interest income | 16,848 | 11,113 | 48,083 | 27,767
Other income | 295 | 110 | 452 | 463
Total investment income | 17,143 | 11,223 | 48,535 | 28,230
Interest and other financing expenses | 9,061 | 5,468 | 26,336 | 11,875
Other expenses | 317 | 205 | 853 | 641
Total expenses | 9,378 | 5,673 | 27,189 | 12,516
Net investment income before income taxes | 7,765 | 5,550 | 21,346 | 15,714
Net realized losses on investments | (545) | (128) | (2,400) | (145)
Net change in unrealized appreciation (depreciation) of investments | 11,856 | (1,566) | 27,810 | (37,004)
Net increase in net assets resulting from operations related to New Mountain Finance Corporation | 19,076 | 3,856 | 46,756 | (21,435)
SLP IV |  |  |  |
Investment Company, Nonconsolidated Subsidiary [Line Items] |  |  |  |
Interest income | 12,476 | 8,333 | 35,669 | 20,749
Other income | 100 | 102 | 258 | 257
Total investment income | 12,576 | 8,435 | 35,927 | 21,006
Interest and other financing expenses | 6,447 | 3,940 | 18,455 | 8,288
Other expenses | 221 | 188 | 653 | 596
Total expenses | 6,668 | 4,128 | 19,108 | 8,884
Net investment income before income taxes | 5,908 | 4,307 | 16,819 | 12,122
Net realized losses on investments | (97) | (85) | (1,751) | (77)
Net change in unrealized appreciation (depreciation) of investments | 10,361 | (1,013) | 19,942 | (25,142)
Net increase in net assets resulting from operations related to New Mountain Finance Corporation | $ 16,172 | $ 3,209 | $ 35,010 | $ (13,097)

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