Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Estimate fair value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	$ 3,900,000	$ 3,900,000
Mortgage loans held for sale	1,518,300,000	1,065,100,000
Mortgage servicing rights	17,100,000
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	31,300,000	19,200,000
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	(16,200,000)	(4,100,000)
Estimate fair value [Member] | Recurring [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments		(1,000,000.0)
Mortgage loans held for sale	1,529,000,000.0	1,072,000,000.0
Mortgage servicing rights	17,100,000	0
Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	0	0
Mortgage loans held for sale	0	0
Mortgage servicing rights	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 1 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments		0
Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	0	0
Mortgage loans held for sale	1,503,200,000	1,055,300,000
Mortgage servicing rights	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	31,300,000	19,200,000
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	(16,200,000)	(4,100,000)
Recurring [Member] | Level 2 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments		(1,000,000.0)
Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	3,900,000	3,900,000	$ 3,900,000
Mortgage loans held for sale	15,100,000	9,800,000	$ 7,800,000
Mortgage servicing rights	17,100,000	0
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
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	0.05
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	0	0
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
Recurring [Member] | Level 3 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives not designated as hedging instruments		$ 0