Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
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
Mortgage loans held for sale	$ 2,021,100,000	1,518,300,000
Mortgage servicing rights	4,100,000	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	17,200,000	31,300,000
Mortgage hedging instruments and commitments	700,000
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	8,500,000	(16,200,000)
Mortgage loans held for sale	2,027,300,000	1,529,000,000
Mortgage servicing rights	4,100,000	17,100,000
Mortgage hedging instruments and commitments	1,700,000	16,500,000
Loans Receivable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net transfers to (out of) Level 3	(400,000)	7,700,000
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
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]
Debt securities collateralized by residential real estate		0
Mortgage loans held for sale	2,009,400,000	1,503,200,000
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	17,200,000	31,300,000
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	8,500,000	(16,200,000)
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	0	3,900,000	$ 3,900,000
Mortgage loans held for sale	11,700,000	15,100,000	9,800,000
Mortgage servicing rights	4,100,000	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	0.11
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	0.06
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