BORROWINGS - Narrative (Details)				1 Months Ended		3 Months Ended		4 Months Ended	6 Months Ended
	Aug. 02, 2023 USD ($)	May 11, 2022	Apr. 20, 2021 USD ($)	Aug. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Aug. 01, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Nov. 22, 2022 USD ($)	Jan. 28, 2022 USD ($)	Nov. 30, 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						229.00%		229.00%	229.00%
Recorded value						$ 732,667,000		$ 732,667,000	$ 732,667,000				$ 637,644,000
Outstanding balance						741,875,000		741,875,000	741,875,000				645,000,000
January 2026 Notes
Debt Instrument [Line Items]
Aggregate principal amount						140,000,000		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				150,000,000
August 2028 Notes
Debt Instrument [Line Items]
Aggregate principal amount						71,875,000		71,875,000	71,875,000				0
SBA Debentures
Debt Instrument [Line Items]
Aggregate principal amount						130,000,000		130,000,000	130,000,000				120,000,000
Notes | January 2026 Notes
Debt Instrument [Line Items]
Recorded value						139,220,000		139,220,000	139,220,000				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%
Outstanding balance						$ 140,000,000		$ 140,000,000	$ 140,000,000				140,000,000
Weighted average interest rate						4.46%		4.46%	4.46%
Interest expense						$ 1,700,000	$ 3,300,000		$ 1,700,000	$ 3,300,000
Average borrowings						140,000,000	140,000,000		140,000,000	140,000,000
Redemption price, percentage					100.00%
Notes | January 2026 Notes | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value						117,300,000		$ 117,300,000	117,300,000
Notes | October 2026 Notes
Debt Instrument [Line Items]
Recorded value						$ 147,633,000		$ 147,633,000	$ 147,633,000				147,263,000
Aggregate principal amount				$ 100,000,000												$ 50,000,000
Fixed interest rate				3.375%												3.375%
Issuance price percent received				99.418%												99.993%
Yield-to-maturity				3.50%		3.50%		3.50%	3.50%							3.375%
Outstanding balance						$ 150,000,000		$ 150,000,000	$ 150,000,000				150,000,000
Interest expense						1,500,000	1,400,000		2,900,000	2,900,000
Average borrowings						150,000,000	$ 150,000,000		150,000,000	$ 150,000,000
Redemption price, percentage				100.00%
Notes | October 2026 Notes | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value						126,100,000		126,100,000	126,100,000
Notes | August 2028 Notes
Debt Instrument [Line Items]
Recorded value						69,438,000		69,438,000	69,438,000
Aggregate principal amount						71,900,000		71,900,000	71,900,000			$ 71,900,000
Fixed interest rate												7.75%
Outstanding balance						$ 71,875,000		$ 71,875,000	$ 71,875,000
Weighted average interest rate						7.75%		7.75%	7.75%
Interest expense						$ 1,500,000			$ 1,800,000
Average borrowings								$ 71,900,000
Notes | August 2028 Notes | Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Fair value						73,600,000		73,600,000	73,600,000
Notes | August 2028 Notes Overallotment
Debt Instrument [Line Items]
Aggregate principal amount												$ 9,400,000
Unsecured Debt | SBA Debentures
Debt Instrument [Line Items]
Recorded value						$ 126,376,000		126,376,000	$ 126,376,000				116,330,000
Weighted-average interest rate						4.23%	2.58%		4.09%	2.55%
Outstanding balance						$ 130,000,000		130,000,000	$ 130,000,000				$ 120,000,000
Interest expense						1,500,000	$ 600,000		2,900,000	$ 1,000,000
Average borrowings						128,200,000	80,000,000		126,000,000	65,900,000
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			87,500,000		87,500,000	87,500,000
Leverage commitment amount														$ 50,000,000	$ 40,000,000		$ 40,000,000
Available leverage commitment						79,000,000		79,000,000	79,000,000
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value						116,200,000		116,200,000	116,200,000
Credit Facility | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 435,000,000										$ 400,000,000
Accordion feature increase limit	$ 750,000,000
Minimum senior coverage ratio		2.00
Maximum asset coverage ratio		1.50
Interest coverage ratio		2.00
Recorded value						250,000,000		250,000,000	250,000,000
Amortization of deferred loan costs						$ 4,300,000	$ 2,900,000		$ 9,300,000	$ 4,800,000
Weighted-average interest rate						7.70%	4.46%		7.52%	3.86%
Average outstanding amount						$ 185,100,000	$ 214,600,000		$ 106,100,000	$ 198,800,000
Outstanding balance						$ 250,000,000		$ 250,000,000	$ 250,000,000
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 | SOFR
Debt Instrument [Line Items]
Basis spread on variable rate	2.15%