Investments - Investment Company, Nonconsolidated Subsidiary, Summarized Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total investment income	$ 387,802	$ 307,125	$ 298,953
Interest and other debt financing expenses	89,378	65,739	74,858
Administrative service fee	7,188	7,227	6,037
Total expenses	193,515	143,453	159,894
Net investment income before taxes	196,191	167,672	139,059
Net realized (gain) loss on investments	19,549	13,324	(18,680)
Net change in unrealized appreciation (depreciation) on investments	(56,806)	160,175	(61,850)
Senior Loan Fund LLC
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest income	0	0	2,800
Fee income	0	0	0
Total investment income	0	0	2,800
Interest and other debt financing expenses	0	0	634
Administrative service fee	0	0	61
Other expenses	0	0	(15)
Total expenses	0	0	680
Net investment income before taxes	0	0	2,120
Net realized (gain) loss on investments	0	0	0
Net change in unrealized appreciation (depreciation) on investments	0	0	(1,603)
Net increase (decrease) in net assets resulting from operations	0	0	517
GCIC Senior Loan Fund LLC
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest income	0	0	2,081
Total investment income	0	0	2,081
Interest and other debt financing expenses	0	0	512
Administrative service fee	0	0	45
Other expenses	0	0	(24)
Total expenses	0	0	533
Net investment income before taxes	0	0	1,548
Net change in unrealized appreciation (depreciation) on investments	0	0	(108)
Net increase (decrease) in net assets resulting from operations	$ 0	$ 0	$ 1,440