Fair Value Measurements Fair Value Measurements - Level 3 Rollforward (Details) - USD ($)	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	$ 2,566,500,000	$ 2,477,500,000
Mortgage servicing rights	27,100,000	5,900,000
Loans Receivable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	3,000,000.0	100,000
Purchases	0	0
Sales and Settlements	(22,600,000)	(900,000)
Principal Reductions	0	0
Net transfers to (out of) Level 3	17,600,000	(200,000)
Contractual Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	800,000	(1,000,000.0)
Purchases	50,800,000	19,200,000
Sales and Settlements	(30,400,000)	(23,400,000)
Principal Reductions	0	0
Net transfers to (out of) Level 3	0	0
Derivative Financial Instruments, Assets \| Derivative Financial Instruments, Liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(2,100,000)	76,100,000
Purchases	0	0
Sales and Settlements	0	0
Principal Reductions	0	0
Net transfers to (out of) Level 3	0	0
Fair Value, Recurring [Member] \| Estimate of Fair Value Measurement [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	2,513,300,000	2,431,800,000
Mortgage servicing rights	27,100,000	5,900,000
Fair Value, Recurring [Member] \| Interest Rate Lock Commitments \| Estimate of Fair Value Measurement [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	42,400,000	44,500,000
Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	8,600,000	10,600,000	$ 11,600,000
Mortgage servicing rights	27,100,000	5,900,000	11,100,000
Level 3 [Member] \| Fair Value, Recurring [Member] \| Interest Rate Lock Commitments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net	42,400,000	44,500,000	$ (31,600,000)
Measurement Input, Prepayment Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	0.06	0.13
Measurement Input, Discount Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	0.11	0.11
Measurement Input, Default Rate [Member] \| Level 3 [Member] \| Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Servicing Asset at Fair Value, Amount	$ 0.05	$ 0.09

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Fair value as of the balance sheet date of all derivatives not designated as hedging instruments.

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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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Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

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