Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Mortgage loans held for sale	$ 796.4	$ 587.3
Homebuilding [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Mortgage loans held for sale	0.0	0.0
Financial Services [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Mortgage loans held for sale	796.4	587.3
Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities collateralized by residential real estate	0.0	0.0
Mortgage loans held for sale	0.0	0.0
Recurring [Member] | Level 1 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Recurring [Member] | Level 1 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Recurring [Member] | Level 1 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities collateralized by residential real estate	0.0	0.0
Mortgage loans held for sale	784.6	580.2
Recurring [Member] | Level 2 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	10.5	9.4
Recurring [Member] | Level 2 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	3.3	1.1
Recurring [Member] | Level 2 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.2	0.6
Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities collateralized by residential real estate	3.9	8.8	$ 0.0
Mortgage loans held for sale	7.8	5.6	$ 6.8
Recurring [Member] | Level 3 [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Recurring [Member] | Level 3 [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Recurring [Member] | Level 3 [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	0.0	0.0
Estimate fair value [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities collateralized by residential real estate	3.9	8.8
Mortgage loans held for sale	792.4	585.8
Estimate fair value [Member] | Recurring [Member] | Interest rate lock commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	10.5	9.4
Estimate fair value [Member] | Recurring [Member] | Forward Sales Of MBS [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	3.3	1.1
Estimate fair value [Member] | Recurring [Member] | Best-efforts and mandatory commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Derivatives not designated as hedging instruments	$ 0.2	$ 0.6