Fair Value Measurements - Recurring (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	$ 658.2	$ 587.3
Fair Value, Measurements, 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	8.8	8.8
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	656.2	585.8
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Interest rate lock commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	18.2	9.4
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Forward sales of MBS [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	(1.8)	1.1
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Best-efforts and mandatory commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	(0.4)	0.6
Fair Value, Measurements, 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
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Interest rate lock commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Forward sales of MBS [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Best-efforts and mandatory commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Measurements, 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
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	651.8	580.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Interest rate lock commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	18.2	9.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Forward sales of MBS [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	(1.8)	1.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Best-efforts and mandatory commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	(0.4)	0.6
Fair Value, Measurements, 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	8.8	8.8	$ 3.9	$ 0.0
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	4.4	5.6	$ 8.4	$ 6.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Interest rate lock commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Forward sales of MBS [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Best-efforts and mandatory commitments [Member]
Fair value balance sheet items measured on a recurring basis
Fair value of interest rate derivatives	0.0	0.0
Homebuilding [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	$ 0.0	$ 0.0