Derivative Financial Instruments - Offsetting Liabilities and Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative financial assets, gross amount	$ 83,631	$ 48,700
Derivative financial assets, amount offset	(5,263)	(2,445)
Derivative financial assets, cash collateral	20,012	12,279
Derivative financial assets, net amount presented in Consolidated Balance Sheets	98,380	58,534
Derivative financial liabilities, gross amount	(64,389)	(38,826)
Derivative financial liabilities, amount offset	5,263	2,445
Derivative financial liabilities, cash collateral	59,028	36,368
Derivative liability, net amount presented on the Consolidated Balance Sheets	(98)	(13)
Collateral held for initial margin	$ 22,900	$ 18,300