Derivative Financial Instruments - Summary of Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative assets, gross amount	$ 90,316	$ 48,700
Derivative assets, fair value offset amount	(6,859)	(2,445)
Derivative assets, cash collateral	22,293	12,279
Derivative Assets	105,750	58,534
Derivative liabilities, gross amount	(71,177)	(38,826)
Derivative liabilities, fair value offset amount	6,859	2,445
Derivative liabilities, cash collateral	64,305	36,368
Derivative Liabilities	(13)	(13)
Collateral held for initial margin	$ 25,200	$ 18,300