Mortgage Loans - Mortgage Loans Held for Sale Textual (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Loans Receivable [Line Items]
Mortgage loans held for sale	$ 2,566.5	$ 2,477.5
Mortgage loans held for sale, principal amount	2,670.0	2,490.0
Payments for Origination of Mortgage Loans Held-for-sale	23,700.0	24,000.0	$ 21,200.0
Proceeds from Sale of Mortgage Loans Held-for-sale	23,500.0	24,000.0	21,000.0
Net gain on sales of loans	566.3	589.9	$ 538.4
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Recurring [Member]
Loans Receivable [Line Items]
Mortgage loans held for sale	2,504.7	2,421.2
Loan Origination Commitments [Member]
Loans Receivable [Line Items]
Derivative, Notional Amount	677.5	637.9
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	$ 1.9	2.4
Other Customer [Member] \| Mortgage Loans \| Customer Concentration Risk
Loans Receivable [Line Items]
Concentration Risk, Percentage	27.00%
Loans Sold to FNMA or securities backed by GNMA [Member] \| Mortgage Loans \| Customer Concentration Risk
Loans Receivable [Line Items]
Concentration Risk, Percentage	71.00%
Uncommitted Loans [Member]
Loans Receivable [Line Items]
Mortgage loans held for sale	$ 2,300.0	$ 1,900.0

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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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