Loans Receivable - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable for loans	$ 77,349	$ 57,070
Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest receivable	Interest receivable
Loans pledged to secure borrowings from the Federal Home Loan Bank	$ 8,941,201	$ 8,224,951
Approximate rate reduction concession, minimum	1.00%
Approximate rate reduction concession, maximum	2.00%
TDR outstanding balance	$ 46,117	56,817
Percent of restructured loans performing	97.90%
Single family residential loans as percentage of restructured loans	84.70%
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction	6 months
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction	12 months
Financing Receivable from Related Party Including Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans granted to officers, directors, and related interests	$ 134,860	$ 144,178