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Summary of Significant Accounting Policies - Narrative (Details) |  |  | 3 Months Ended | 9 Months Ended | 11 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 01, 2024 USD ($) $ / shares shares | Mar. 30, 2020 USD ($) $ / shares shares | Sep. 30, 2024 USD ($) agreement | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) agreement | Sep. 30, 2023 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2023 numberOfAgreement | Dec. 31, 2023 agreement | Feb. 04, 2016 USD ($)
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of collateralized agreements |  |  | 1 |  | 1 |  |  |  | 1 | 1 |
Securities purchased under collateralized agreements to resell, cost |  |  | $ 30,000,000 |  | $ 30,000,000 |  |  | $ 30,000,000 |  |  |
Securities purchased under agreements to resell |  |  | 13,500,000 |  | 13,500,000 |  |  | 16,500,000 |  |  |
Cash equivalents |  |  | 0 |  | 0 |  |  | 0 |  |  |
PIK interest income |  |  | 9,050,000 | $ 7,917,000 | 27,735,000 | $ 24,807,000 |  |  |  |  |
Non-cash dividend income |  |  | $ 8,319,000 | 6,988,000 | $ 23,484,000 | 20,225,000 |  |  |  |  |
Investments threshold period past due for nonaccrual status |  |  | 30 days |  | 30 days |  |  |  |  |  |
Deferred tax liability |  |  | $ 1,100,000 |  | $ 1,100,000 |  |  | 0 |  |  |
Deferred tax asset |  |  | $ 0 |  | $ 0 |  |  | $ 594,000 |  |  |
Dividend reinvestment plan, share price percentage, threshold |  |  | 110.00% |  | 110.00% |  |  |  |  |  |
Investments in Majority-owned Subsidiaries |  |  |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Shares redeemed for a promissory note (in shares) | shares |  | 105,030 |  |  |  |  |  |  |  |  |
Redemption of shares paid-in-kind |  | $ 11,315,000 |  |  |  |  |  |  |  |  |
Interest rate |  | 7.00% |  |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  |  |  |  | $ 50,000,000
Common stock repurchased |  |  | $ 0 | $ 0 | $ 0 | $ 0 | $ 2,948,000 |  |  |  |
Common Stock | New Mountain Net Lease Corporation |  |  |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sale of stock, number of shares issued (in shares) | shares |  | 105,030 |  |  |  |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | $ / shares |  | $ 107.73 |  |  |  |  |  |  |  |  |
Sale of stock, consideration received |  | $ 11,315,000 |  |  |  |  |  |  |  |  |
Common Stock | New Mountain Net Lease Corporation | Investments in Majority-owned Subsidiaries |  |  |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | $ / shares | $ 73.39 |  |  |  |  |  |  |  |  |  |
Sale of stock, consideration received | $ 4,666,000 |  |  |  |  |  |  |  |  |  |
Shares sold (in shares) | shares | 63,575 |  |  |  |  |  |  |  |  |  |
Common Stock | Affiliate Of Investment Adviser | New Mountain Net Lease Corporation |  |  |  |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sale of stock, number of shares issued (in shares) | shares | 63,575 |  |  |  |  |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | $ / shares | $ 73.39 |  |  |  |  |  |  |  |  |  |
Sale of stock, consideration received | $ 4,666,000 |  |  |  |  |  |  |  |  |  |

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