Income Taxes - Reconciliation of Net Increase in Net Assets Resulting from Operations to Taxable Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2022	Aug. 31, 2021	Aug. 31, 2020
Income Tax Disclosure [Abstract]
Net increase (decrease) in net assets resulting from operations	$ 735,343	$ 428,106	$ (78,949)
Net realized (gains) losses on investments	22,375	16,173	10,139
Net unrealized (gains) losses on investments	(405,414)	(143,654)	328,997
Other temporary book-to-tax differences	(76,361)	(47,330)	(91,368)
Permanent differences	30	(20)	57
Taxable income before deductions for distributions	$ 275,973	$ 253,275	$ 168,876