BORROWINGS - Narrative (Details)					1 Months Ended		3 Months Ended					6 Months Ended	12 Months Ended
	Mar. 20, 2024 USD ($)	Aug. 02, 2023 USD ($)	Sep. 24, 2021 USD ($)	Apr. 20, 2021 USD ($)	Aug. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2024 USD ($) facility	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Sep. 30, 2021 USD ($)	Sep. 24, 2021 USD ($)	Mar. 31, 2024 USD ($) facility	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 20, 2023 USD ($)	Dec. 07, 2023 USD ($) lender	Dec. 06, 2023 USD ($)	Aug. 01, 2023 USD ($)	Nov. 22, 2022 USD ($)	Jan. 28, 2022 USD ($)	Nov. 30, 2021 USD ($)	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							221.00%						221.00%
Long-term line of credit							$ 265,000,000						$ 265,000,000	$ 235,000,000
Interest													43,088,000	28,873,000	$ 19,924,000
Realized loss on extinguishment of debt							0	$ 0	$ (361,000)	$ 0			(361,000)	0	(17,087,000)
Notes | October 2024 Notes
Debt Instrument [Line Items]
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%
Realized loss on disposal of fixed assets											$ 1,800,000
Interest expense													$ 0	$ 0	$ 3,600,000
Average borrowings												$ 125,000,000
Realized loss on extinguishment of debt											$ 15,200,000
Weighted average effective interest rate															5.375%
Notes | January 2026 Notes
Debt Instrument [Line Items]
Weighted average effective interest rate													4.46%	4.46%	4.46%
Interest													$ 6,637,000	$ 6,644,000	$ 6,661,000
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
Issuance price percent received																								102.11%
Yield-to-maturity																								4.00%
Redemption price, percentage						100.00%
Notes | October 2026 Notes
Debt Instrument [Line Items]
Weighted average effective interest rate													3.50%	3.50%	3.50%
Interest													$ 5,876,000	$ 5,796,000	$ 3,093,000
Aggregate principal amount					$ 100,000,000																	$ 50,000,000
Fixed interest rate					3.375%																	3.375%
Average borrowings													$ 150,000,000	$ 150,000,000	$ 132,900,000
Issuance price percent received					99.418%																	99.993%
Yield-to-maturity					3.50%																	3.375%
Redemption price, percentage					100.00%
Notes | August 2028 Notes
Debt Instrument [Line Items]
Weighted average effective interest rate													7.75%	0.00%	0.00%
Interest													$ 4,848,000	$ 0	$ 0
Aggregate principal amount										$ 71,900,000
Fixed interest rate										7.75%
Average borrowings													$ 71,875,000	$ 0	$ 0
Notes | August 2028 Notes Overallotment
Debt Instrument [Line Items]
Aggregate principal amount										$ 9,400,000
Unsecured Debt | SBA Debentures
Debt Instrument [Line Items]
Weighted average effective interest rate													4.18%	3.38%	1.30%
Interest													$ 6,037,000	$ 3,218,000	$ 346,000
Average borrowings													129,708,000	$ 82,641,000	$ 16,971,000
Unsecured Debt | SBA Debentures | SBIC I
Debt Instrument [Line Items]
Maturity				10 years
Regulatory maximum borrowing capacity				$ 175,000,000
Regulatory capital				$ 87,500,000			87,500,000						87,500,000
Leverage commitment amount							175,000,000						175,000,000			$ 45,000,000				$ 50,000,000	$ 40,000,000		$ 40,000,000
Available leverage commitment							87,500,000						87,500,000
Unused commitments							$ 22,000,000						$ 22,000,000
Corporate Credit Facility | Line of Credit
Debt Instrument [Line Items]
Weighted average effective interest rate													7.66%
Average borrowings													$ 217,500,000
Corporate Credit Facility | Line of Credit | Corporate Credit Facility
Debt Instrument [Line Items]
Number of revolving credit facility | facility							1						1
Weighted average effective interest rate													7.66%	5.22%	2.50%
Average borrowings													$ 217,500,000	$ 213,658,000	$ 173,500,000
Maximum borrowing capacity		$ 435,000,000															$ 460,000,000	$ 435,000,000	$ 400,000,000
Accordion feature increase limit		$ 750,000,000															$ 25,000,000
Number of lenders | lender																	10
Minimum senior coverage ratio		2.00
Maximum asset coverage ratio		1.50
Interest coverage ratio		2.00
Interest													$ 19,626,000	$ 13,152,000	$ 6,158,000
Corporate Credit Facility | Line of Credit | Corporate Credit Facility | Minimum
Debt Instrument [Line Items]
Commitment fee percentage		0.50%
Corporate Credit Facility | Line of Credit | Corporate Credit Facility | Maximum
Debt Instrument [Line Items]
Commitment fee percentage		1.00%
Corporate Credit Facility | Line of Credit | SOFR | Corporate Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate		2.15%
Secured Debt | Line of Credit | SPV Credit Facility
Debt Instrument [Line Items]
Number of revolving credit facility | facility							1						1
Maximum borrowing capacity	$ 150,000,000
Accordion feature increase limit	200,000,000
Accordion feature that allows increases (up to)	$ 400,000,000
Long-term line of credit							$ 0						$ 0
Interest													$ 45,000
Secured Debt | Line of Credit | SPV Credit Facility | Before April 20 2024
Debt Instrument [Line Items]
Commitment fee percentage	0.10%
Secured Debt | Line of Credit | SPV Credit Facility | After April 20 2024
Debt Instrument [Line Items]
Commitment fee percentage	0.35%
Secured Debt | Line of Credit | SOFR | SPV Credit Facility | Before March 20 2027
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Secured Debt | Line of Credit | SOFR | SPV Credit Facility | After March 20 2027
Debt Instrument [Line Items]
Basis spread on variable rate	2.85%