BORROWINGS - ING Capital Credit Facility (Details) € in Millions, £ in Millions, $ in Millions		3 Months Ended	12 Months Ended
	Jan. 15, 2021 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Mar. 31, 2024 GBP (£)	Mar. 31, 2024 EUR (€)	Mar. 31, 2024 AUD ($)	Dec. 31, 2023 GBP (£)	Dec. 31, 2023 EUR (€)	Dec. 31, 2023 AUD ($)	Oct. 13, 2022 USD ($)	Apr. 25, 2022 USD ($)	Mar. 31, 2022 USD ($)	Jul. 22, 2021 USD ($)	Apr. 30, 2021 USD ($)
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities		$ 642,422,000	$ 644,463,000
Letter of Credit
Debt Instrument [Line Items]
Commitment aggregate amount														$ 25,000,000
Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross		642,422,000	644,463,000
ING Capital Credit Facility | Line of Credit
Debt Instrument [Line Items]
Commitment aggregate amount	$ 65,000,000									$ 710,000,000	$ 625,000,000	$ 500,000,000	$ 500,000,000	$ 325,000,000
Line of credit facility, accordion feature, aggregate limit											$ 800,000,000
Variable rate	1.00%
Floor rate	0.00%
Asset coverage ratio, if assets to portfolio loans is more than 70%	150.00%
ING Capital Credit Facility | Line of Credit | Minimum
Debt Instrument [Line Items]
Asset coverage ratio, if assets to portfolio loans is equal or less than 70%	167.00%
ING Capital Credit Facility | Line of Credit | Maximum
Debt Instrument [Line Items]
Asset coverage ratio, if assets to portfolio loans is equal or less than 70%	200.00%
ING Capital Credit Facility | Line of Credit | One-Month Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate	1.00%
ING Capital Credit Facility | Line of Credit | One-Month Secured Overnight Financing Rate (SOFR) | Interest Period Of One Month
Debt Instrument [Line Items]
Debt instrument, credit spread adjustment	0.0010
ING Capital Credit Facility | Line of Credit | Federal Funds Purchased
Debt Instrument [Line Items]
Variable rate	0.50%
ING Capital Credit Facility | Line of Credit | Overnight Bank Funding Rate
Debt Instrument [Line Items]
Variable rate	0.50%
ING Capital Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Variable rate	2.15%
Floor rate	0.00%
ING Capital Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Interest Period Of One Month
Debt Instrument [Line Items]
Debt instrument, credit spread adjustment	0.0010
ING Capital Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Interest Period Of Three Month
Debt Instrument [Line Items]
Debt instrument, credit spread adjustment	0.0015
ING Capital Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Interest Period Of Six Month
Debt Instrument [Line Items]
Debt instrument, credit spread adjustment	0.0025
ING Capital Credit Facility Denominated In US Dollars | Line of Credit | Base Rate
Debt Instrument [Line Items]
Variable rate	1.15%
ING Capital Credit Facility Denominated In Pounds Sterling, Swiss Francs, Euros, Canadian Dollars, Danish Krone, Norwegian Krone or Swedish Krona | Line of Credit | Adjusted Eurocurrency Rate
Debt Instrument [Line Items]
Variable rate	2.15%
ING Capital Credit Facility Denominated In Australian Dollars | Line of Credit | Adjusted Eurocurrency Rate
Debt Instrument [Line Items]
Variable rate	2.35%
ING Capital Credit Facility Denominated In New Zealand Dollars | Line of Credit | Adjusted Eurocurrency Rate
Debt Instrument [Line Items]
Variable rate	2.45%
Borrowings Denominated In U.S Dollars under ING Credit Facility | Line of Credit
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities		$ 533,000,000.0
Effective interest rate		7.569%		7.569%	7.569%	7.569%
Borrowings Denominated In U.S Dollars under ING Credit Facility | Line of Credit | ING Capital Credit Facility
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities			$ 533,000,000
Effective interest rate			7.584%				7.584%	7.584%	7.584%
Borrowings Denominated In U.S Dollars under ING Credit Facility | Line of Credit | ING Capital Credit Facility | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Variable rate		5.319%	5.334%
Borrowings Denominated In British Pounds | Line of Credit | ING Capital Credit Facility
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities		$ 39,400,000	$ 39,800,000	£ 31.2			£ 31.2
Effective interest rate		7.371%	7.37%	7.371%	7.371%	7.371%	7.37%	7.37%	7.37%
Borrowings Denominated In British Pounds | Line of Credit | ING Capital Credit Facility | Sterling Overnight Index Average (SONIA)
Debt Instrument [Line Items]
Variable rate		5.221%	5.22%
Borrowings Denominated In Euro | Line of Credit | ING Capital Credit Facility
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities		$ 66,400,000	$ 67,900,000		€ 61.5			€ 61.5
Effective interest rate		6.088%	6.025%	6.088%	6.088%	6.088%	6.025%	6.025%	6.025%
Borrowings Denominated In Euro | Line of Credit | ING Capital Credit Facility | Euro Interbank Offered Rate (EURIBOR)
Debt Instrument [Line Items]
Variable rate		3.938%	3.875%
Borrowings Denominated In Australian Dollars | Line of Credit | ING Capital Credit Facility
Debt Instrument [Line Items]
Borrowings under subscription and credit facilities		$ 3,600,000	$ 3,800,000			$ 5.5			$ 5.5
Effective interest rate		6.65%	6.65%	6.65%	6.65%	6.65%	6.65%	6.65%	6.65%
Borrowings Denominated In Australian Dollars | Line of Credit | ING Capital Credit Facility | AUD Screen Rate
Debt Instrument [Line Items]
Variable rate		4.30%	4.30%