Fair Value Measurements Fair Value Measurements - Level 3 Rollforward (Details) - USD ($)	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	$ 2,477,500,000	$ 2,519,900,000
Mortgage servicing rights	5,900,000	11,100,000
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	100,000	2,600,000
Purchases	0	0
Sales and Settlements	(900,000)	(14,800,000)
Principal Reductions	0	0
Net transfers to (out of) Level 3	(200,000)	9,700,000
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	(1,000,000.0)	200,000
Purchases	19,200,000	52,400,000
Sales and Settlements	(23,400,000)	(52,100,000)
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	76,100,000	104,200,000
Purchases	0	0
Sales and Settlements	0	0
Principal Reductions	0	0
Net transfers to (out of) Level 3	0	0
Level 3 [Member] | Fair Value, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Mortgage loans held for sale	10,600,000	11,600,000	$ 14,100,000
Mortgage servicing rights	5,900,000	11,100,000	10,600,000
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	44,500,000	(31,600,000)	$ (135,800,000)
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	0.13	0.10
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	$ 0.09	$ 0.08