BORROWINGS - Narrative (Details)			1 Months Ended		3 Months Ended		7 Months Ended	9 Months Ended
	Aug. 02, 2023 USD ($)	Apr. 20, 2021 USD ($)	Aug. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 20, 2023 USD ($)	Dec. 07, 2023 USD ($) lender	Dec. 06, 2023 USD ($)	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					253.00%		253.00%	253.00%
Recorded value					$ 677,835,000		$ 677,835,000	$ 677,835,000							$ 637,644,000
Outstanding balance					686,875,000		686,875,000	686,875,000							645,000,000
Notes | January 2026 Notes
Debt Instrument [Line Items]
Recorded value					139,304,000		139,304,000	139,304,000							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	$ 5,000,000		$ 1,700,000	$ 5,000,000
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					121,200,000		$ 121,200,000	121,200,000
Notes | October 2026 Notes
Debt Instrument [Line Items]
Recorded value					$ 147,884,000		$ 147,884,000	$ 147,884,000							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		4,400,000	4,300,000
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					130,500,000		130,500,000	130,500,000
Notes | August 2028 Notes
Debt Instrument [Line Items]
Recorded value					69,566,000		69,566,000	69,566,000
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			$ 3,300,000
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					73,300,000		73,300,000	73,300,000
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					$ 126,081,000		126,081,000	$ 126,081,000							116,330,000
Weighted-average interest rate					4.24%	3.70%		4.13%	2.98%
Outstanding balance					$ 130,000,000		130,000,000	$ 130,000,000							$ 120,000,000
Interest expense					1,500,000	$ 900,000		4,400,000	$ 1,900,000
Average borrowings					130,000,000	83,900,000		127,300,000	54,100,000
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	87,500,000
Leverage commitment amount					175,000,000		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	87,500,000
Unused commitments					45,000,000		45,000,000	45,000,000
Unsecured Debt | SBA Debentures | Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Fair value					122,800,000		122,800,000	122,800,000
Credit Facility | Line of Credit
Debt Instrument [Line Items]
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
Recorded value					195,000,000		195,000,000	195,000,000
Amortization of deferred loan costs					$ 5,200,000	$ 3,900,000		$ 14,500,000	$ 8,800,000
Weighted-average interest rate					7.88%	6.02%		7.61%	4.63%
Average outstanding amount					$ 224,700,000	$ 228,000,000		$ 162,900,000	$ 208,600,000
Outstanding balance					$ 195,000,000		$ 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 | SOFR
Debt Instrument [Line Items]
Basis spread on variable rate	2.15%