Borrowings - Schedule of Information about Senior Securities (Parenthetical) (Details) - USD ($)	2 Months Ended	5 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2017	Jun. 30, 2022	Sep. 30, 2023	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Apr. 30, 2021	Oct. 28, 2020	Sep. 30, 2017
Debt Instrument [Line Items]
Asset coverage per unit measure per dollar			$ 1,000,000	$ 1,000,000
Revolving Credit Facilities
Debt Instrument [Line Items]
Aggregate principal amount				74,000,000	$ 49,321,000	$ 79,571,000							$ 78,000,000	$ 79,000,000	$ 92,000,000	$ 80,571,000
7.375% Notes Due 2019
Debt Instrument [Line Items]
Interest Rate			7.375%
Average market value per par value	$ 1,012,280							$ 1,016,040.00	$ 1,000,000	$ 1,011,960	$ 1,037,720	$ 1,032,960
6.125% Notes Due 2022
Debt Instrument [Line Items]
Interest Rate			6.125%														6.125%
Average market value per par value		$ 1,006,000			953,200	1,009,930	$ 1,009,200
6.125% Notes Due 2022 \| Unsecured Debt
Debt Instrument [Line Items]
Interest Rate																			6.125%
Aggregate principal amount					76,726,000	$ 77,407,000													$ 77,400,000
4.875% Notes Due 2026 \| Unsecured Debt
Debt Instrument [Line Items]
Interest Rate																	4.875%
Aggregate principal amount				108,000,000									108,000,000	108,000,000	108,000,000	108,000,000
2018-2 Secured Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 139,493,000	$ 251,863,000								$ 147,672,000	$ 171,288,000	$ 178,163,000	$ 163,863,000		$ 312,000,000