Borrowings - Schedule of Information about Senior Securities (Parenthetical) (Details) - USD ($)	5 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2025	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 15, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Mar. 28, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Apr. 30, 2021	Oct. 28, 2020
Debt Instrument [Line Items]
Asset coverage per unit measure per dollar		$ 1,000,000
Aggregate principal amount												$ 267,479,000
Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate principal amount		130,779,000	$ 49,321,000	$ 79,571,000						$ 147,379,000	$ 147,379,000	159,479,000		$ 92,000,000	$ 92,000,000	$ 80,571,000
KeyBank Credit Facility
Debt Instrument [Line Items]
Aggregate principal amount		$ 33,332,000
7.375% Notes Due 2019
Debt Instrument [Line Items]
Interest Rate		7.375%
Average market value per par value						$ 1,016,040.00	$ 1,000,000	$ 1,011,960
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]
Aggregate principal amount			76,726,000	$ 77,407,000
4.875% Notes due 2026
Debt Instrument [Line Items]
Aggregate principal amount												108,000,000
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	108,000,000	108,000,000
2018-2 Secured Notes
Debt Instrument [Line Items]
Aggregate principal amount		0	$ 251,863,000									$ 0		$ 125,683,000	$ 178,163,000	$ 163,863,000		$ 312,000,000
2026 Notes
Debt Instrument [Line Items]
Interest Rate									5.25%				6.00%
Aggregate principal amount		50,000,000
2032 Convertible Notes
Debt Instrument [Line Items]
Interest Rate									6.00%				5.25%
Aggregate principal amount		$ 2,500,000