Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Estimate fair value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	$ 2,431,800,000	$ 2,480,700,000
Mortgage servicing rights	5,900,000	11,100,000
Interest rate lock commitments	44,500,000	2,300,000
Mortgage hedging instruments and commitments	2,800,000	153,600,000
Interest Rate Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value		(33,900,000)
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(63,000,000.0)	(15,700,000)
Estimate fair value [Member] | Recurring [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	44,500,000	(31,600,000)
Estimate fair value [Member] | Recurring [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	(60,200,000)	137,900,000
Mortgage loans held for sale	2,477,500,000	2,519,900,000
Mortgage servicing rights	5,900,000	11,100,000
Mortgage hedging instruments and commitments	2,800,000	153,600,000
Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate lock commitments	44,500,000	2,300,000
Interest Rate Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	0	(33,900,000)
Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	0	0
Mortgage servicing rights	0	0
Recurring [Member] | Level 1 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	0	0
Recurring [Member] | Level 1 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	0	0
Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	2,421,200,000	2,469,100,000
Mortgage servicing rights	0	0
Recurring [Member] | Level 2 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	0	0
Recurring [Member] | Level 2 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	(60,200,000)	137,900,000
Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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
Recurring [Member] | Level 3 [Member] | Measurement Input, Prepayment Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset at Fair Value, Amount	0.13	0.10
Recurring [Member] | Level 3 [Member] | Measurement Input, Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset at Fair Value, Amount	0.11	0.11
Recurring [Member] | Level 3 [Member] | Measurement Input, Default Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset at Fair Value, Amount	0.09	0.08
Recurring [Member] | Level 3 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	44,500,000	(31,600,000)	$ (135,800,000)
Recurring [Member] | Level 3 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments	$ 0	$ 0