Taxable/Distributable Income and Dividend Distributions - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Investments, Owned, Federal Income Tax Note [Line Items]
Undistributed ordinary income, net	$ 699,900
Valuation allowance	6,200
Deferred tax assets	6,500
Investment income, operating, tax expense (benefit), realized and unrealized gains (loss)	900	$ 100	$ 1,700
Unrealized gain (loss), investment and derivative, operating, tax expense (benefit)	400
Deferred taxes	269	$ (5)	$ 1,670
Current income tax	100
Cost of investments	3,088,800
Unrealized appreciation	758,800
Unrealized depreciation	1,000,600
Unrealized appreciation (depreciation)	241,800
Short-Term Capital Loss Carryforward
Investments, Owned, Federal Income Tax Note [Line Items]
Undistributed ordinary income, net	62,800
Long-Term Capital Loss Carryforward
Investments, Owned, Federal Income Tax Note [Line Items]
Undistributed ordinary income, net	$ 637,100