Mortgage Loans - Mortgage Loans Held for Sale Textual (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Loans Receivable [Line Items]
Payments for Origination of Mortgage Loans Held-for-sale	$ 24,000.0	$ 21,200.0	$ 19,500.0
Proceeds from Sale of Mortgage Loans Held-for-sale	24,000.0	21,000.0	18,900.0
Net gain on sales of loans	589.9	538.4	$ 561.7
Mortgage loans held for sale	2,477.5	2,519.9
Mortgage loans held for sale, principal amount	2,500.0	2,600.0
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Loans Receivable [Line Items]
Mortgage loans held for sale	2,421.2	2,469.1
Loan Origination Commitments [Member]
Loans Receivable [Line Items]
Derivative, Notional Amount	637.9	1,100.0
Loan Origination Commitments [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Loans Receivable [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	$ 2.4	15.7
Other Customer [Member] \| Mortgage Loans \| Customer Concentration Risk
Loans Receivable [Line Items]
Concentration Risk, Percentage	26.00%
Loans Sold to FNMA or securities backed by GNMA [Member] \| Mortgage Loans \| Customer Concentration Risk
Loans Receivable [Line Items]
Concentration Risk, Percentage	73.00%
Uncommitted Loans [Member]
Loans Receivable [Line Items]
Mortgage loans held for sale	$ 1,900.0	$ 1,700.0

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The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

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Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

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The amount of cash paid for the origination of mortgages that are held for sale.

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The cash inflow from sales of loans that are secured with real estate mortgages and are held with the intention to resell in the near future.

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