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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
Mar. 31, 2021
Sep. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Mortgage loans held for sale $ 1,755,800,000 $ 1,529,000,000.0
Other Assets, Miscellaneous 42,900,000 39,900,000
Other Accrued Liabilities 97,400,000 84,400,000
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt securities collateralized by residential real estate 3,900,000 3,900,000
Mortgage loans held for sale 1,745,500,000 1,518,300,000
Servicing Asset at Fair Value, Amount 6,400,000 17,100,000
Fair Value, Recurring [Member] | Interest rate lock commitments [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net (17,300,000) 31,300,000
Other Assets, Miscellaneous 26,800,000  
Other Accrued Liabilities 44,100,000  
Fair Value, Recurring [Member] | Forward sales of MBS [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net 87,800,000 (16,200,000)
Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt securities collateralized by residential real estate 0 0
Mortgage loans held for sale 0 0
Servicing Asset at Fair Value, Amount 0 0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | Interest rate lock commitments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net 0 0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | Forward sales of MBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net 0 0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt securities collateralized by residential real estate 0 0
Mortgage loans held for sale 1,727,000,000.0 1,503,200,000
Servicing Asset at Fair Value, Amount 0 0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | Interest rate lock commitments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net (17,300,000) 31,300,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | Forward sales of MBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net 87,800,000 (16,200,000)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt securities collateralized by residential real estate 3,900,000 3,900,000
Mortgage loans held for sale 18,500,000 15,100,000
Servicing Asset at Fair Value, Amount 6,400,000 17,100,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Measurement Input, Prepayment Rate    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Servicing Asset at Fair Value, Amount 0.10 0.13
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Measurement Input, Discount Rate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Servicing Asset at Fair Value, Amount 0.11 0.11
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Measurement Input, Default Rate    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Servicing Asset at Fair Value, Amount 0.06 0.05
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Interest rate lock commitments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net 0 0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | Forward sales of MBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Interest Rate Derivative Instruments Not Designated as Hedging Instruments at Fair Value, Net $ 0 $ 0