Income Taxes - Narrative (Details) - USD ($) $ in Thousands				3 Months Ended			12 Months Ended
	Jun. 30, 2022	Aug. 25, 2021	Mar. 24, 2020	Sep. 30, 2022	Sep. 30, 2021		Aug. 31, 2022	Aug. 31, 2021	Jun. 30, 2021	Aug. 31, 2020
Income Taxes [Line Items]
Ordinary income		$ 265,593					$ 230,743	$ 251,171		$ 169,041
Return of capital		$ 12,263	$ 1,500		$ 3,695	[1]	0	25,784		96,720
Distributable loss	$ (68,360)			$ 107,911					$ 210,570
Reclassification of distributions from capital									22,089
Increase to distributable earning					$ 3,695
Tax exempt dividends				53.20%
Section 163 qualified dividends				81.55%
Section 243 and 854 qualified dividends				4.61%
Cumulative taxable income and capital gain in excess of cumulative distributions.							$ 21,453
Cost basis of investments for tax purposes	7,214,493			$ 7,541,133
Unrealized appreciation (depreciation), net				41,532
Tax basis of investments, gross, unrealized appreciation	388,017			1,423,888
Tax basis of investments, gross, unrealized depreciation				1,382,356
Unrealized gain on investments	1,506,944
Unrealized loss on investments	1,118,927
Overdistributed net investment income adjustment, increase (decrease)								30		(20)
Additional paid in capital	$ 4,050,370			$ 4,071,937				$ (30)		$ 20
Previously Reported
Income Taxes [Line Items]
Distributable loss									$ 232,659

[1] Certain reclassifications have been made in the presentation of prior year amounts since initial disclosure in our Quarterly Report on Form 10-Q for the period ended September 30, 2021 to conform to the presentation in our Annual Report on Form 10-K for the year ended June 30, 2022. In addition, we have not yet finalized return of capital estimates for the current period. See Note 2 and Note 12 within the accompanying notes to consolidated financial statements for further discussion.