Allowance for Loan Losses - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision (release) for credit losses	$ 3,000,000	$ 500,000	$ 21,750,000
Recoveries, net of charge-offs	3,508,000	6,345,000
Allowance for loan losses	172,808,000	171,300,000	166,955,000
SBA PPP loans	16,113,564,000	13,833,570,000
Loans Subject to Quantitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	115,245,000
Loans Subject to Management Qualitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	57,563,000
Non-Performing Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, gross	$ 44,554,000	$ 43,625,000
Ratio of non-performing assets to total assets	0.21%	0.22%
Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	$ 34,534,000	$ 31,749,000
Commercial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	133,476,000	125,375,000	109,923,000
Commercial loans | Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	16,296,000	11,723,000
Commercial loans | Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	57,210,000	45,957,000	$ 38,665,000
Commercial loans | Commercial & industrial | Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	5,693,000	$ 365,000
Commercial loans | Commercial & industrial | SBA Payroll Protection Program, CARES Act
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	$ 0