Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision (release) for credit losses	$ 41,500	$ 3,000	$ 500
Allowance for credit loss, charge-off (recovery)	45,101	(3,508)
Allowance for loan losses	177,207	172,808	171,300
Loans Subject to Quantitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	107,049
Loans Subject to Management Qualitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	70,158
Non-Performing Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, gross	$ 57,924	$ 44,554
Ratio of non-performing assets to total assets	0.26%	0.21%
Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	$ 50,422	$ 34,534
Commercial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	137,194	133,476	125,375
Commercial loans | Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	34,644	16,296
Commercial loans | Commercial & industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	58,773	57,210	$ 45,957
Commercial loans | Commercial & industrial | Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual	$ 6,082	$ 5,693