Allowance for Losses on Loans - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provisions & Transfers	$ 6,500,000	$ (500,000)	$ 10,000,000	$ (2,500,000)
Net charge-offs	5,877,000	(473,000)	5,388,000	(2,584,000)
Allowance for credit losses on loans	177,420,000	171,384,000	177,420,000	171,384,000	$ 176,797,000	$ 172,808,000	$ 171,411,000	$ 171,300,000
Net loans	17,271,906,000		17,271,906,000			16,113,564,000
Gross loans	$ 17,449,326,000		$ 17,449,326,000			$ 16,286,372,000
% based on $, past due	0.16%		0.16%			0.17%
Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 34,606,000		$ 34,606,000			$ 34,534,000
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 46,785,000		$ 46,785,000			$ 44,554,000
Ratio of non-performing assets to total assets	0.21%		0.21%			0.21%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provisions & Transfers	$ 5,380,000	1,522,000	$ 10,216,000	2,432,000
Allowance for credit losses on loans	137,665,000	129,440,000	137,665,000	129,440,000	138,282,000	$ 133,476,000	127,837,000	125,375,000
Gross loans	$ 10,479,087,000		$ 10,479,087,000			$ 9,728,501,000
% based on $, past due	0.07%		0.07%			0.08%
Commercial loans | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 18,044,000		$ 18,044,000			$ 16,296,000
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provisions & Transfers	5,440,000	7,543,000	12,153,000	8,798,000
Allowance for credit losses on loans	63,301,000	$ 53,903,000	63,301,000	$ 53,903,000	$ 63,873,000	57,210,000	$ 47,231,000	$ 45,957,000
Gross loans	$ 2,585,196,000		$ 2,585,196,000			$ 2,343,403,000
% based on $, past due	0.16%		0.16%			0.28%
Commercial loans | Commercial & industrial | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 7,681,000		$ 7,681,000			$ 5,693,000
Commercial loans | Commercial & industrial | SBA Payroll Protection Program, CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans	0		0			0
Net loans	$ 2,781,000		$ 2,781,000			$ 10,141,399