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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
Dec. 31, 2020
Sep. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Mortgage loans held for sale $ 1,438,900,000 $ 1,529,000,000.0
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 1,429,500,000 1,518,300,000
Servicing Asset at Fair Value, Amount 9,800,000 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 26,400,000 31,300,000
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 (38,500,000) (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 1,411,600,000 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 26,400,000 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 (38,500,000) (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 17,900,000 15,100,000
Servicing Asset at Fair Value, Amount 9,800,000 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 0.11 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 0.05 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