Federal Income Tax Matters - Reconciliation of Taxable Income Increase (Decrease) in Net Assets from Operation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Schedule of Investments [Abstract]
Net increase (decrease) in net assets resulting from operations	$ 258,631	$ 153,440	$ 340,280
Net change in unrealized (appreciation) depreciation on investment transactions	(13,555)	61,898	(165,246)
Other income not currently taxable	(22,437)	(7,574)	(13,466)
Expenses not currently deductible	11,198	18,652	33,125
Other income for tax but not book	3,773	5,846	8,215
Other deductions/losses for tax not book	(39,639)	(519)	(5,295)
Other realized gain/loss differences	29,175	41,313	(2,762)
Taxable income before deductions for distributions	$ 227,146	$ 273,056	$ 194,851