BORROWINGS - Narrative (Details)					1 Months Ended		3 Months Ended
	May 11, 2022 USD ($)	Aug. 09, 2021 USD ($) lender	Aug. 08, 2021 USD ($) lender	Apr. 20, 2021 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2023 USD ($)	Nov. 22, 2022 USD ($)	Nov. 16, 2022 USD ($) lender	May 10, 2022 USD ($)	Jan. 28, 2022 USD ($)	Nov. 30, 2021 USD ($)	Aug. 31, 2021 USD ($)	May 25, 2021 USD ($)	Feb. 28, 2021 USD ($)	Apr. 25, 2019
Debt Instrument [Line Items]
Asset coverage ratio																		166.00%
Current asset coverage ratio					237.00%		237.00%
Recorded value					$ 672,262,000		$ 672,262,000		$ 637,644,000
Outstanding balance					681,875,000		681,875,000		645,000,000
January 2026 Notes
Debt Instrument [Line Items]
Aggregate principal amount					140,000,000		140,000,000		140,000,000
October 2026 Notes
Debt Instrument [Line Items]
Aggregate principal amount					150,000,000		150,000,000		150,000,000
August 2028 Notes
Debt Instrument [Line Items]
Aggregate principal amount					71,875,000		71,875,000		0
SBA Debentures
Debt Instrument [Line Items]
Aggregate principal amount					125,000,000		125,000,000		120,000,000
Notes | January 2026 Notes
Debt Instrument [Line Items]
Recorded value					$ 139,135,000		$ 139,135,000		139,051,000
Aggregate principal amount						$ 75,000,000											$ 65,000,000
Fixed interest rate						4.50%
Issuance price percent received																	102.11%
Yield-to-maturity																	4.00%
Weighted average interest rate					4.46%		4.46%
Interest expense							$ 1,700,000	$ 1,700,000
Average borrowings							140,000,000	140,000,000
Redemption price, percentage						100.00%
Outstanding balance					$ 140,000,000		140,000,000		140,000,000
Notes | January 2026 Notes | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value					118,400,000		118,400,000
Notes | October 2026 Notes
Debt Instrument [Line Items]
Recorded value					$ 147,448,000		$ 147,448,000		147,263,000
Aggregate principal amount														$ 50,000,000	$ 100,000,000
Fixed interest rate														3.375%	3.375%
Issuance price percent received														99.993%	99.418%
Yield-to-maturity					3.50%		3.50%							3.375%	3.50%
Interest expense							$ 1,500,000	1,400,000
Average borrowings							150,000,000	$ 150,000,000
Redemption price, percentage						100.00%
Outstanding balance					$ 150,000,000		150,000,000		150,000,000
Notes | October 2026 Notes | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value					127,800,000		127,800,000
Notes | August 2028 Notes
Debt Instrument [Line Items]
Recorded value					69,327,000		69,327,000
Aggregate principal amount					$ 71,900,000		$ 71,900,000
Fixed interest rate					7.75%		7.75%
Weighted average interest rate					7.75%		7.75%
Interest expense							$ 300,000
Average borrowings					$ 71,900,000
Outstanding balance					71,875,000		71,875,000
Notes | August 2028 Notes | Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Fair value					72,300,000		72,300,000
Notes | August 2028 Notes Overallotment
Debt Instrument [Line Items]
Aggregate principal amount					9,400,000		9,400,000
Unsecured Debt | SBA Debentures
Debt Instrument [Line Items]
Recorded value					121,352,000		$ 121,352,000		116,330,000
Weighted-average interest rate							3.94%	2.50%
Interest expense							$ 1,400,000	$ 400,000
Average borrowings							123,700,000	51,600,000
Outstanding balance					125,000,000		125,000,000		$ 120,000,000
Unsecured Debt | SBA Debentures | SBIC I
Debt Instrument [Line Items]
Debt instrument term				10 years
Regulatory maximum borrowing capacity				$ 175,000,000
Regulatory capital				$ 87,500,000	65,000,000		65,000,000
Leverage commitment amount					130,000,000		130,000,000			$ 50,000,000			$ 40,000,000			$ 40,000,000
Available leverage commitment					65,000,000		65,000,000
Unused commitments					5,000,000		5,000,000
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value					117,800,000		117,800,000
Credit Facility | Line of Credit
Debt Instrument [Line Items]
LIBOR floor		0.00%	0.00%
Maximum borrowing capacity	$ 380,000,000	$ 335,000,000	$ 340,000,000								$ 400,000,000	$ 335,000,000
Number of lenders | lender		10	11								11
Covenants increase the minimum obligors		$ 200,000,000	$ 180,000,000
Accordion feature, higher borrowing capacity option	$ 400,000,000
Accordion feature increase limit											$ 20,000,000
Minimum senior coverage ratio	2
Maximum asset coverage ratio	1.50
Interest coverage ratio	2.25
Outstanding advances exceed percentage	90.00%
Minimum liquidity	0.10
Recorded value					195,000,000		195,000,000
Amortization of deferred loan costs							$ 4,900,000	$ 1,900,000
Weighted-average interest rate							7.36%	3.16%
Average outstanding amount							$ 239,700,000	$ 182,900,000
Outstanding balance					$ 195,000,000		$ 195,000,000
Credit Facility | Line of Credit | Minimum
Debt Instrument [Line Items]
Commitment fee percentage	0.50%
Credit Facility | Line of Credit | Maximum
Debt Instrument [Line Items]
Commitment fee percentage	1.00%
Credit Facility | Line of Credit | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		2.15%	2.50%