Borrowings - Narrative (Details) - USD ($)	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Borrowings	$ 3,650,000,000	$ 2,125,000,000
Weighted average effective maturity of FHLB advances	2 years 3 days
FRB Bank Term Funding Program
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Debt instrument, face amount	$ 750,000,000
Debt term	1 year
Federal Home Loan Bank of Des Moines
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal home loan bank, percent of assets	45.00%

Amount of short-term and long-term debt and lease obligation.

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