Fair Value Measurements - Assets and Liabilities Not Reflected at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	$ 920.3	$ 1,007.8	$ 1,150.3	$ 1,303.2
Restricted Cash and Cash Equivalents	53.7	16.5
Notes payable	3,258.1	2,871.6
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	920.3	1,007.8
Restricted cash, Fair value	53.7	16.5
Notes payable, Fair value	3,378.0	3,015.2
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	920.3	1,007.8
Restricted cash, Fair value	53.7	16.5
Notes payable, Fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	2,679.5	2,584.1
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	698.5	431.1
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	558.0	973.0
Restricted Cash and Cash Equivalents	8.3	9.3
Notes payable	$ 2,749.6	$ 2,451.6