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Borrowings - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Federal Home Loan Bank, Advance, Branch of FHLBank [Line Items]
Borrowings	$ 3,267,589,000	$ 3,650,000,000
Weighted average effective maturity of FHLB advances	2 years 4 months 2 days
FRB Bank Term Funding Program
Federal Home Loan Bank, Advance, Branch of FHLBank [Line Items]
Issued amount	$ 325,000,000	$ 750,000,000
Debt term	1 year
Federal Home Loan Bank of Des Moines
Federal Home Loan Bank, Advance, Branch of FHLBank [Line Items]
Federal home loan bank, percent of assets	45.00%

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