Nature of Operations and Summary of Significant Accounting Policies - Narrative (Details)			3 Months Ended	12 Months Ended
		Oct. 28, 2020 $ / shares	Mar. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) portfolio_class segment $ / shares	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of reportable segments | segment				1
Dividends per share (in dollars per share) | $ / shares				$ 0.76	$ 1.10	$ 0.90
Maturity period of loans and loan commitments				5 years
Number of loan portfolio classes | portfolio_class				6
FDIC Indemnification, expected reimbursement rate (as a percent)				80.00%
FDIC, Clawback Liability, payment period following termination or maturity of agreement				45 days
Residential real estate loans in process of foreclosure				$ 500,000	$ 600,000
Impairment of long-lived assets				0	0	$ 0
Impairment of goodwill				740,562,000	0	0
Impairment of intangible assets				1,800,000	0	0
(Benefit from) provision for income taxes				$ (25,510,000)	$ 48,230,000	74,119,000
Reclassification from AOCI to retained earnings	[1]					0
Subsequent Event
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dividends per share (in dollars per share) | $ / shares		$ 0.01
Accumulated Other Comprehensive (Loss) Income
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification from AOCI to retained earnings			$ 2,400,000			$ (2,353,000)	[1]
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity period of loans and loan commitments				5 years
Minimum | Buildings and building improvements
Accounts, Notes, Loans and Financing Receivable [Line Items]
Premises and equipment, useful life				10 years
Minimum | Furniture and equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Premises and equipment, useful life				3 years
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity period of loans and loan commitments				15 years
Maximum | Buildings and building improvements
Accounts, Notes, Loans and Financing Receivable [Line Items]
Premises and equipment, useful life				40 years
Maximum | Furniture and equipment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Premises and equipment, useful life				10 years

[1] Reclassification due to adoption of ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.