Fair Value Measurements - Recurring (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale	$ 1,379.4	$ 1,072.0
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	3.9	3.9
Mortgage loans held for sale	1,370.7	1,065.1
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	64.4	19.2
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Forward sales of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	(69.1)	(4.1)
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	(11.7)	(1.0)
Fair Value, Recurring [Member] | Fair Value, Inputs, 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	0.0
Mortgage loans held for sale	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Forward sales of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, 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	0.0
Mortgage loans held for sale	1,356.6	1,055.3
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	64.4	19.2
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Forward sales of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	(69.1)	(4.1)
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	(11.7)	(1.0)
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities collateralized by residential real estate	3.9	3.9
Mortgage loans held for sale	14.1	9.8
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Forward sales of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of interest rate derivatives	$ 0.0	$ 0.0