Derivative Financial Instruments - Effect on the Consolidated Statement of Comprehensive Income (Details) - Noninterest income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Interest rate swaps and other derivatives
Derivative [Line Items]
Amount of Loss Recognized in Consolidated Statements of Income	$ (3,681)	$ (20,904)	$ (40,379)	$ (50,252)
Mortgage loan commitments
Derivative [Line Items]
Amount of Loss Recognized in Consolidated Statements of Income	(627)	23	21	44
Mortgage loan forward sale contracts
Derivative [Line Items]
Amount of Loss Recognized in Consolidated Statements of Income	$ 627	$ (23)	$ (21)	$ (44)