Derivative Financial Instruments - Effect on the Consolidated Statement of Comprehensive Income (Details) - Noninterest income - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Interest rate swaps
Derivative [Line Items]
Amount of gain (loss) recognized in income	$ (38,439)	$ (63,444)	$ 44,596
Mortgage loan commitments
Derivative [Line Items]
Amount of gain (loss) recognized in income	107	17	(28)
Mortgage loan forward sale contracts
Derivative [Line Items]
Amount of gain (loss) recognized in income	$ (107)	$ (17)	$ 28

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109998890-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration