BORROWINGS - Narrative (Details)						1 Months Ended		3 Months Ended		6 Months Ended
	May 11, 2022 USD ($)	Sep. 24, 2021 USD ($)	Aug. 09, 2021 USD ($) lender	Aug. 08, 2021 USD ($) lender	Apr. 20, 2021 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	May 10, 2022 USD ($)	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								205.00%		205.00%
Recorded Value								$ 603,329,000		$ 603,329,000			$ 528,588,000
Realized loss on extinguishment of debt								0	$ (17,087,000)	0	$ (17,087,000)
Outstanding Balance								610,000,000		610,000,000			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								80,000,000		80,000,000			40,000,000
Notes | January 2026 Notes
Debt Instrument [Line Items]
Recorded Value								138,883,000		138,883,000			138,714,000
Interest expense								1,700,000	1,700,000	3,300,000	3,400,000
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								120,600,000		120,600,000
Notes | October 2026 Notes
Debt Instrument [Line Items]
Recorded Value								146,893,000		146,893,000			146,522,000
Interest expense								1,400,000	400,000	2,900,000	400,000
Aggregate principal amount															$ 50,000,000	$ 100,000,000
Fixed Interest Rate															3.375%	3.375%
Average borrowings						$ 100,000,000		$ 150,000,000		$ 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								129,700,000		129,700,000
Notes | October 2024 Notes
Debt Instrument [Line Items]
Interest expense									1,700,000		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
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									$ 115,500,000		$ 120,200,000
Weighted average interest rate						5.375%			5.375%		5.375%
Unsecured Debt | SBA Debentures
Debt Instrument [Line Items]
Recorded Value								77,553,000		77,553,000			38,352,000
Interest expense								$ 600,000	$ 46,000	$ 1,000,000	$ 53,700
Weighted average interest rate								2.58%	0.85%	2.55%	0.84%
Average borrowings								$ 80,000,000	$ 9,200,000	$ 65,900,000	$ 4,600,000
Debt instrument term					10 years
Regulatory maximum borrowing capacity					$ 175,000,000
Regulatory capital					$ 87,500,000
Leverage commitment amount														$ 40,000,000			$ 40,000,000
Outstanding Balance								80,000,000		80,000,000			$ 40,000,000
Unsecured Debt | SBA Debentures | SBIC I
Debt Instrument [Line Items]
Regulatory capital								65,000,000		65,000,000
Available leverage commitment								50,000,000		50,000,000
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value								73,800,000		73,800,000
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								$ 335,000,000
Number of lenders | lender			10	11
Covenants increase the minimum obligors			$ 200,000,000	$ 180,000,000
Accordion feature increase limit	$ 400,000,000
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								240,000,000		240,000,000
Amortization of deferred loan costs								2,900,000		4,800,000
Interest expense								$ 1,500,000		$ 2,900,000
Weighted average interest rate								4.46%	2.58%	3.86%	2.64%
Average outstanding amount								$ 214,600,000	$ 159,600,000	$ 198,800,000	$ 148,500,000
Outstanding Balance								$ 240,000,000		$ 240,000,000
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%