Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investments, foreign currency translation and forward currency contracts	$ (61,898)	$ 165,246	$ (65,527)
Level 3
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investments, foreign currency translation and forward currency contracts	$ (119,448)	$ 68,612	$ (93,152)