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Debt - Additional Information (Details) - USD ($) $ in Thousands |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
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Aug. 17, 2022 | Nov. 02, 2021 | Oct. 08, 2021 | Feb. 16, 2021 | Jan. 19, 2021 | Dec. 23, 2020 | Jun. 26, 2020 | Oct. 23, 2019 | Apr. 24, 2019 | Feb. 19, 2019 | Feb. 02, 2018 | Sep. 30, 2022 | Mar. 31, 2022 | Sep. 30, 2021 | Mar. 31, 2021 | Sep. 30, 2022 | Sep. 30, 2021 | Dec. 31, 2021 | Oct. 16, 2019 | Feb. 08, 2019 | Feb. 22, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument asset coverage percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 277.70% |  | 282.20% |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 133,000 |  |  |  | $ 133,000 |  | $ 107,000 |  |  |
Weighted average interest rate |  |  |  |  |  |  |  |  |  |  |  | 3.921% |  | 4.239% |  | 3.921% | 4.239% |  |  |  |
Offering expenses |  |  |  |  |  |  |  |  |  |  |  | $ 9,805 |  |  |  | $ 9,805 |  | 8,601 |  |  |
Loss on extinguishment of debt |  |  |  |  |  |  |  |  |  |  |  | 53 | $ 198 | $ 460 | $ 2,180 | 251 | $ 2,640 |  |  |  |
Secured borrowings |  |  |  |  |  |  |  |  |  |  |  | 16,995 |  |  |  | 16,995 |  | 17,637 |  |  |
Aggregate amount of outstanding of senior securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 266,995 |  | 267,637 |  |  |
Fair value of loans included in investments |  |  |  |  |  |  |  |  |  |  |  | $ 16,910 |  |  |  | $ 16,910 |  | 17,522 |  |  |
Credit facility, interest rate per annum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.20% | 0.50% |  |  |  |
SBA Debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  |  |  |  |  |  |  | 3.214% |  | 2.899% |  | 3.214% | 2.899% |  |  |  |
Offering expenses |  |  |  |  |  |  |  |  |  |  |  | $ 4,197 |  |  |  | $ 4,197 |  | 3,022 |  |  |
Prepayment of debentures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,000 | $ 63,500 |  |  |  |
Debt instrument, maturity year range, start |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2025 | 2025 |  |  |  |
Debt instrument, maturity year range, end |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2028 | 2028 |  |  |  |
Loss on extinguishment of debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (251) | $ (2,640) |  |  |  |
Debt instrument term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |
Interest rate payment term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | semi-annually on March 1 and September 1 |  |  |  |  |
Unused commitments amount |  |  |  |  |  |  |  |  |  |  |  | $ 57,000 |  |  |  | $ 57,000 |  | $ 63,000 |  |  |
Secured Borrowings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate |  |  |  |  |  |  |  |  |  |  |  | 6.446% |  | 4.392% |  | 6.446% | 4.392% | 4.40% |  |  |
5.875% Notes Due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  | $ 43,478 |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  |  | 5.875% |  |  |  |  |  |  |  |  |  |
Debt instrument additional aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,522
Net proceeds from the notes |  |  |  |  |  |  |  |  |  |  | $ 48,062 |  |  |  |  |  |  |  |  |  |
Underwriting discounts |  |  |  |  |  |  |  |  |  |  | 1,500 |  |  |  |  |  |  |  |  |  |
Offering expenses |  |  |  |  |  |  |  |  |  |  | $ 438 |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of aggregate principal amount |  |  |  |  | $ 50,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt |  |  |  |  | $ (794) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
6.000% Notes Due 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 69,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 60,000 |
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |
Debt instrument additional aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,000 |
Net proceeds from the notes |  |  |  |  |  |  |  |  |  | $ 66,521 |  |  |  |  |  |  |  |  |  |  |
Underwriting discounts |  |  |  |  |  |  |  |  |  | 2,070 |  |  |  |  |  |  |  |  |  |  |
Offering expenses |  |  |  |  |  |  |  |  |  | $ 409 |  |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of aggregate principal amount |  |  |  | 50,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of remaining aggregate principal amount |  | $ 19,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt |  | (313) |  | $ (1,081) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
5.375% Notes Due 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55,000 |  |
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.375% |  |
Debt instrument additional aggregate principal amount |  |  |  |  |  |  |  | $ 8,250 |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the notes |  |  |  |  |  |  |  | 61,053 |  |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discounts |  |  |  |  |  |  |  | 1,898 |  |  |  |  |  |  |  |  |  |  |  |  |
Offering expenses |  |  |  |  |  |  |  | $ 300 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of aggregate principal amount |  | 63,250 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt |  | $ (1,311) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
4.75% Notes due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity date |  |  |  |  |  | Jan. 31, 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | $ 125,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  | 4.75% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the notes |  |  |  |  |  | $ 122,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discounts |  |  |  |  |  | 2,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Offering expenses |  |  |  |  |  | $ 400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, redemption description |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | The January 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed more than three months prior to maturity and at par thereafter. Interest on the January 2026 Notes is payable on January 31 and July 31 of each year. The Company does not intend to list the January 2026 Notes on any securities exchange or automated dealer quotation system. |  |  |  |  |
3.50% Notes Due 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity date |  |  | Nov. 15, 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 125,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  | 3.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from the notes |  |  | $ 122,095 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discounts |  |  | $ 2,505 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of public offering price |  |  | 99.996% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Offering expenses |  |  | $ 400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, redemption description |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | The November 2026 Notes may be redeemed in whole or in part at any time or from time to time at our option subject to a make whole provision if redeemed more than three months prior to maturity and at par thereafter. Interest on the November 2026 Notes is payable on May 15 and November 15 of each year. The Company does not intend to list the November 2026 Notes on any securities exchange or automated dealer quotation system. |  |  |  |  |
2023 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of issued and outstanding amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000 |  |  |  |
2024 Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument redemption of issued and outstanding amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000 |  |  |  |
Revolving Credit Facility | Credit Agreement | ING Capital LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity date | Aug. 17, 2027 |  |  |  |  |  | Apr. 24, 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate after certain conditions satisfied | 2.675% |  |  |  |  |  | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Asset coverage ratio | 1.50% |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior asset coverage ratio | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining unused portion of Credit Facility commitment percentage |  |  |  |  |  |  |  |  | 0.50% |  |  |  |  |  |  |  |  |  |  |  |
Performing first lien bank loans, minimum percentage |  |  |  |  |  |  |  |  | 35.00% |  |  |  |  |  |  |  |  |  |  |  |
Revolving Credit Facility | Credit Agreement | ING Capital LLC | SOFR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate prior to certain conditions satisfied | 3.00% |  |  |  |  |  | 2.675% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving Credit Facility | Credit Agreement | ING Capital LLC | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility, interest rate per annum |  |  |  |  |  |  |  |  | 2.675% |  |  |  |  |  |  |  |  |  |  |  |
Credit facility commitment percentage |  |  |  |  |  |  |  |  | 35.00% |  |  |  |  |  |  |  |  |  |  |  |
Revolving Credit Facility | Credit Agreement | ING Capital LLC | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility, interest rate per annum |  |  |  |  |  |  |  |  | 2.50% |  |  |  |  |  |  |  |  |  |  |  |

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