Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Estimate fair value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	$ 2,480,700,000	$ 2,370,200,000
Mortgage servicing rights	11,100,000	10,600,000
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	(31,600,000)	(135,800,000)
Mortgage hedging instruments and commitments	(33,900,000)	(183,500,000)
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	137,900,000	317,800,000
Mortgage loans held for sale	2,519,900,000	2,386,000,000
Mortgage servicing rights	11,100,000	10,600,000
Mortgage hedging instruments and commitments	153,600,000	330,200,000
Mortgage hedging instruments and commitments	(15,700,000)	(12,400,000)
Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate lock commitments	2,300,000	47,700,000
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	2,469,100,000	2,356,100,000
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	137,900,000	317,800,000
Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	11,600,000	14,100,000	$ 11,700,000
Mortgage servicing rights	11,100,000	10,600,000	4,100,000
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	0.10
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
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	0.08
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	(31,600,000)	(135,800,000)	$ 17,200,000
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