BORROWINGS - Narrative (Details)						1 Months Ended	3 Months Ended				4 Months Ended	6 Months Ended	9 Months Ended
	May 11, 2022 USD ($)	Sep. 24, 2021 USD ($)	Aug. 09, 2021 USD ($) lender	Aug. 08, 2021 USD ($) lender	Apr. 20, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Sep. 24, 2021 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Nov. 22, 2022 USD ($)	Nov. 16, 2022 USD ($) lender	May 10, 2022 USD ($)	Mar. 31, 2022 USD ($)	Jan. 28, 2022 USD ($)	Nov. 30, 2021 USD ($)	Aug. 31, 2021 USD ($)	May 25, 2021 USD ($)	Feb. 28, 2021 USD ($)	Aug. 31, 2020 USD ($)	Oct. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Apr. 25, 2019
Debt Instrument [Line Items]
Asset coverage ratio																											166.00%
Current asset coverage ratio							229.00%						229.00%
Recorded Value							$ 611,627,000						$ 611,627,000					$ 528,588,000
Realized loss on extinguishment of debt							0	$ 0	$ 0	$ (17,087,000)			0	$ (17,087,000)
Outstanding Balance							619,000,000						619,000,000					535,000,000
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
SBA Debentures
Debt Instrument [Line Items]
Aggregate principal amount							104,000,000						104,000,000					40,000,000
Notes | January 2026 Notes
Debt Instrument [Line Items]
Recorded Value							138,967,000						138,967,000					138,714,000
Interest expense							1,700,000		1,600,000				5,000,000	5,000,000
Aggregate principal amount						$ 75,000,000																	$ 65,000,000
Fixed Interest Rate						4.50%
Average borrowings							$ 140,000,000		140,000,000				$ 140,000,000	140,000,000
Weighted average interest rate							4.46%						4.46%
Issuance price percent received																							102.11%
Yield-to-maturity																							4.00%
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							121,900,000						121,900,000
Notes | October 2026 Notes
Debt Instrument [Line Items]
Recorded Value							147,078,000						147,078,000					146,522,000
Interest expense							1,400,000		1,300,000				4,300,000	1,700,000
Aggregate principal amount																				$ 50,000,000	$ 100,000,000
Fixed Interest Rate																				3.375%	3.375%
Average borrowings							$ 150,000,000		128,800,000		$ 120,900,000		$ 150,000,000
Issuance price percent received																				99.993%	99.418%
Yield-to-maturity							3.50%						3.50%							3.375%	3.50%
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							130,000,000						130,000,000
Notes | October 2024 Notes
Debt Instrument [Line Items]
Interest expense									$ 0					$ 3,600,000
Aggregate principal amount																								$ 50,000,000	$ 10,000,000	$ 65,000,000
Fixed Interest Rate																										5.375%
Repurchased face amount		$ 125,000,000										$ 125,000,000
Percentage of principal amount redeemed		100.00%
Unamortized debt issuance costs										1,800,000
Realized loss on extinguishment of debt										$ 15,200,000
Average borrowings												$ 125,000,000
Weighted average interest rate									5.375%		5.375%			5.375%
Unsecured Debt | SBA Debentures
Debt Instrument [Line Items]
Recorded Value							100,582,000						100,582,000					38,352,000
Interest expense							$ 900,000		$ 100,000				$ 1,900,000	$ 200,000
Weighted-average interest rate							3.70%		1.43%				2.98%	1.24%
Average borrowings							$ 83,900,000		$ 21,000,000				$ 54,100,000	$ 10,100,000
Outstanding Balance							104,000,000						104,000,000					$ 40,000,000
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							26,000,000						26,000,000
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value							97,400,000						97,400,000
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							225,000,000						225,000,000
Amortization of deferred loan costs							$ 3,900,000						$ 8,800,000
Interest expense									$ 1,600,000					$ 4,500,000
Weighted-average interest rate							6.02%		2.37%				4.63%	2.52%
Average outstanding amount							$ 228,000,000		$ 200,200,000				$ 208,600,000	$ 165,800,000
Outstanding Balance							$ 225,000,000						$ 225,000,000
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%