Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2022	Sep. 30, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses	$ 2,500	$ 500
Recoveries, net of charge-offs	489	2,111
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	176,797	171,411	$ 172,808	$ 171,300
Net loans	16,993,588		16,113,564
Gross loans	$ 17,170,385		$ 16,286,372
% based on $, past due	0.16%		0.17%
Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 29,180		$ 34,534
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 38,650		$ 44,554
Ratio of non-performing assets to total assets	0.18%		0.21%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	$ 138,282	127,837	$ 133,476	125,375
Gross loans	$ 10,358,994		$ 9,728,501
% based on $, past due	0.03%		0.08%
Commercial loans | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 11,420		$ 16,296
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	63,873	$ 47,231	57,210	$ 45,957
Gross loans	$ 2,621,266		$ 2,343,403
% based on $, past due	0.05%		0.28%
Commercial loans | Commercial & industrial | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 906		$ 5,693