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Prepayment of debentures.

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Secured borrowings.

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Senior asset coverage ratio.

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Face (par) amount of debt instrument at time of issuance.

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Debt - Additional Information (Details) - USD ($) $ in Thousands												3 Months Ended				9 Months Ended		12 Months Ended
	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%