Federal Income Tax Matters - Reconciliation of Taxable Income Increase (Decrease) in Net Assets from Operation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investments [Abstract]
Net increase (decrease) in net assets resulting from operations	$ 153,440	$ 340,280	$ 54,872
Net change in unrealized (appreciation) depreciation on investment transactions	61,898	(165,246)	65,527
Other income not currently taxable	(7,574)	(13,466)	(5,573)
Expenses not currently deductible	18,652	33,125	41,295
Other income for tax but not book	5,846	8,215	9,092
Other deductions/losses for tax not book	(519)	(5,295)	(2,091)
Other realized gain/loss differences	41,313	(2,762)	18,610
Taxable income before deductions for distributions	$ 273,056	$ 194,851	$ 181,732