Fair Value Measurements - Recurring (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Homebuilding [Member] | 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	$ 3.9
Homebuilding [Member] | 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
Homebuilding [Member] | 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
Homebuilding [Member] | 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	3.9	$ 0.0	$ 0.0	$ 33.9
Financial Services and Other [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	577.8	654.0
Financial Services and Other [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	569.4	647.7
Financial Services and Other [Member] | 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	18.0	9.3
Financial Services and Other [Member] | 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	(4.9)	(2.6)
Financial Services and Other [Member] | 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	(1.0)	(0.2)
Financial Services and Other [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	0.0	0.0
Financial Services and Other [Member] | 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
Financial Services and Other [Member] | 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
Financial Services and Other [Member] | 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
Financial Services and Other [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	561.0	640.9
Financial Services and Other [Member] | 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	18.0	9.3
Financial Services and Other [Member] | 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	(4.9)	(2.6)
Financial Services and Other [Member] | 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	(1.0)	(0.2)
Financial Services and Other [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value balance sheet items measured on a recurring basis
Mortgage loans held for sale	8.4	6.8	$ 15.3	$ 13.9
Financial Services and Other [Member] | 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
Financial Services and Other [Member] | 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
Financial Services and Other [Member] | 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