Borrowings - Schedule of Senior Securities Outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount outstanding	$ 519,250,000	$ 559,250,000	$ 81,000,000	$ 99,000,000	$ 61,000,000	$ 59,500,000	$ 0
Asset Coverage per Unit	2,054,000	2,030,000	8,484,000	5,710,000	7,169,000	3,813,000
2027 Senior Notes
Debt Instrument [Line Items]
Amount outstanding	152,250,000	152,250,000
Asset Coverage per Unit	4,593,000	4,784,000
2026 Senior Notes
Debt Instrument [Line Items]
Amount outstanding	95,000,000	70,000,000	20,000,000
Asset Coverage per Unit	6,759,000	9,229,000	31,310,000
Credit Facility
Debt Instrument [Line Items]
Amount outstanding	272,000,000	337,000,000	61,000,000	99,000,000	61,000,000
Asset Coverage per Unit	$ 3,011,000	$ 2,709,000	$ 10,938,000	$ 5,710,000	$ 7,169,000
Credit Facility - CIBC
Debt Instrument [Line Items]
Amount outstanding						59,500,000
Asset Coverage per Unit						$ 3,813,000