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Summary of Significant Accounting Policies - Narrative (Details) |  | 3 Months Ended | 9 Months Ended | 11 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 30, 2020 USD ($) $ / shares shares | Sep. 30, 2023 USD ($) numberOfAgreement | Sep. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) numberOfAgreement | Sep. 30, 2022 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2022 USD ($) numberOfAgreement | Feb. 04, 2016 USD ($)
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |
Number of collateralized agreements | numberOfAgreement |  | 1 |  | 1 |  |  | 1 |
Cost basis of securities purchased under collateralized agreements to resell |  | $ 30,000,000 |  | $ 30,000,000 |  |  | $ 30,000,000 |
Fair value of securities purchased under collateralized agreements to resell |  | 16,500,000 |  | 16,500,000 |  |  | 16,539,000 |
Cash equivalents |  | 0 |  | 0 |  |  | 0 |
Investment income, interest, paid-in-kind |  | 7,917,000 | $ 6,193,000 | 24,807,000 | $ 21,993,000 |  |  |
Investment income, dividend, paid-in-kind |  | $ 6,988,000 | 5,995,000 | $ 20,225,000 | 16,338,000 |  |  |
Investments threshold period past due for nonaccrual status |  | 30 days |  | 30 days |  |  |  |
Income tax provision (benefit) |  | $ (918,000) | (43,000) | $ 335,000 | 122,000 |  |  |
Current income tax expense (benefit) |  | (627,000) | (13,000) | 401,000 | (5,000) |  |  |
Deferred income tax provision (benefit) |  | (291,000) | (30,000) | (66,000) | 127,000 |  |  |
Deferred tax liability |  | $ 2,790,000 |  | $ 2,790,000 |  |  | $ 8,487,000 |
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 |  |  |  |  |  |  |
Investments in Majority-owned Subsidiaries | Debt Investments |  |  |  |  |  |  |  |
Securities Financing Transaction [Line Items] |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |

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