BORROWINGS - Narrative (Details)							1 Months Ended	3 Months Ended			6 Months Ended
	Dec. 09, 2024 USD ($)	Nov. 04, 2024 USD ($) trading_day $ / shares	Mar. 20, 2024 USD ($)	Mar. 01, 2024	Aug. 02, 2023 USD ($)	Apr. 20, 2021 USD ($)	Aug. 31, 2021 USD ($)	Sep. 30, 2025 USD ($) lender facility $ / shares	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) facility lender consecutive_trading_day $ / shares	Sep. 30, 2024 USD ($)	Jun. 20, 2024 USD ($)	Jun. 19, 2024 USD ($)	Aug. 01, 2023 USD ($)	Jun. 30, 2023 USD ($)	Nov. 30, 2021 USD ($)	Feb. 28, 2021 USD ($)	Dec. 31, 2020 USD ($)	Apr. 25, 2019
Debt Instrument [Line Items]
Asset coverage ratio																				166.00%
Current asset coverage ratio								208.00%			208.00%
January 2026 Notes | Notes
Debt Instrument [Line Items]
Weighted average interest rate								0.00%		4.46%	0.00%	4.46%
Aggregate principal amount																		$ 65,000,000	$ 75,000,000
Fixed interest rate																			4.50%
Issuance price percent received																		102.11%
Yield-to-maturity																		4.00%
Repurchased face amount	$ 140,000,000
Redemption price, percentage	100.00%
Realized loss on disposal of fixed assets									$ 400,000
October 2026 Notes | Notes
Debt Instrument [Line Items]
Weighted average interest rate								3.50%		3.50%	3.50%	3.50%
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%										3.375%
Redemption price, percentage							100.00%				100.00%
October 2026 Notes | Convertible Debt
Debt Instrument [Line Items]
Fixed interest rate		5.125%
August 2028 Notes | Notes
Debt Instrument [Line Items]
Weighted average interest rate								7.75%		7.75%	7.75%	7.75%
Aggregate principal amount																$ 71,900,000
Fixed interest rate																7.75%
August 2028 Notes Overallotment | Notes
Debt Instrument [Line Items]
Aggregate principal amount																$ 9,400,000
2029 Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Weighted average interest rate								5.13%		0.00%	5.13%	0.00%
Aggregate principal amount		$ 230,000,000
Fixed interest rate		5.125%
Redemption price, percentage											100.00%
Conversion rate		0.04									40.43
Conversion price (in usd per share) | $ / shares		$ 25.00						$ 24.73			$ 24.73
Dividend increase (in usd per share) | $ / shares								$ 0.58			$ 0.58
Percentage of stock price		130.00%
Trading days | trading_day		20
Consecutive trading days | consecutive_trading_day											30
2029 Convertible Notes | Convertible Debt | Maximum
Debt Instrument [Line Items]
Conversion rate											0.0447828
2029 Convertible Notes Overallotement | Convertible Debt
Debt Instrument [Line Items]
Aggregate principal amount		$ 30,000,000
September 2030 Notes | Notes
Debt Instrument [Line Items]
Weighted average interest rate								5.95%		0.00%	5.95%	0.00%
Aggregate principal amount								$ 350,000,000			$ 350,000,000
Fixed interest rate								5.95%			5.95%
Issuance price percent received								99.345%			99.345%
Yield-to-maturity								6.104%			6.104%
Redemption price, percentage							100.00%
SBA Debentures | Unsecured Debt
Debt Instrument [Line Items]
Weighted average interest rate								4.45%		4.43%	4.43%	4.41%
SBA Debentures | Unsecured Debt | 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
Available leverage commitment								87,500,000			87,500,000
Leverage commitment amount								175,000,000			175,000,000
SBA Debentures | Unsecured Debt | SBIC II
Debt Instrument [Line Items]
Regulatory capital								87,500,000			87,500,000
Available leverage commitment								26,000,000			26,000,000
Leverage commitment amount								$ 40,000,000			$ 40,000,000
Corporate Credit Facility | Line of Credit
Debt Instrument [Line Items]
Weighted average interest rate								6.79%			6.76%
Average borrowings								$ 315,400,000			$ 317,500,000
Corporate Credit Facility | Corporate Credit Facility | Line of Credit
Debt Instrument [Line Items]
Number of revolving credit facility | facility								1			1
Weighted average interest rate								6.72%		7.74%	6.68%	7.75%
Average borrowings								$ 202,391,000		$ 166,685,000	$ 205,328,000	$ 178,579,000
Basis spread on variable rate					2.15%
Maximum borrowing capacity					$ 435,000,000			$ 510,000,000			$ 510,000,000				$ 400,000,000
Accordion feature increase limit					$ 750,000,000
Number of lenders | lender								11			11
Minimum senior coverage ratio				2.00
Maximum asset coverage ratio				1.50
Interest coverage ratio				2.00
Corporate Credit Facility | Corporate Credit Facility | Line of Credit | Minimum
Debt Instrument [Line Items]
Commitment fee percentage				0.50%
Corporate Credit Facility | Corporate Credit Facility | Line of Credit | Maximum
Debt Instrument [Line Items]
Commitment fee percentage				1.00%
SPV Credit Facility | SPV Credit Facility | Line of Credit
Debt Instrument [Line Items]
Number of revolving credit facility | facility								1			1
Weighted average interest rate								6.92%		7.85%	6.90%	7.87%
Average borrowings								$ 112,967,000		$ 70,826,000	$ 112,202,000	$ 58,617,000
Maximum borrowing capacity			$ 150,000,000											$ 150,000,000
Accordion feature increase limit													$ 200,000,000
Accordion feature that allows increases (up to)			$ 400,000,000
SPV Credit Facility | SPV Credit Facility | Line of Credit | Before March 20 2027
Debt Instrument [Line Items]
Basis spread on variable rate			2.50%
SPV Credit Facility | SPV Credit Facility | Line of Credit | After March 20 2027
Debt Instrument [Line Items]
Basis spread on variable rate			2.85%
SPV Credit Facility | SPV Credit Facility | Line of Credit | Before April 20 2024
Debt Instrument [Line Items]
Commitment fee percentage			0.10%
SPV Credit Facility | SPV Credit Facility | Line of Credit | After April 20 2024
Debt Instrument [Line Items]
Commitment fee percentage			0.35%