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Fair Value Measurements - Assets and Liabilities Not Reflected at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	$ 993.2	$ 1,303.2	$ 1,224.1	$ 1,383.8
Notes payable, Carrying value	3,222.4	3,271.3
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	947.9	1,271.8
Restricted cash, Carrying value	11.1	9.5
Notes payable, Carrying value	2,803.4	2,798.3
Homebuilding [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Carrying value	2,788.8	2,786.7
Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Carrying value	14.6	11.6
Homebuilding [Member] \| Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	947.9	1,271.8
Restricted cash, Fair value	11.1	9.5
Homebuilding [Member] \| Estimate of Fair Value Measurement [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	2,941.4	2,947.4
Homebuilding [Member] \| Estimate of Fair Value Measurement [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	14.6	11.6
Homebuilding [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	947.9	1,271.8
Restricted cash, Fair value	11.1	9.5
Homebuilding [Member] \| Fair Value, Inputs, Level 1 [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	0.0	0.0
Homebuilding [Member] \| Fair Value, Inputs, Level 1 [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	0.0	0.0
Homebuilding [Member] \| 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
Homebuilding [Member] \| Fair Value, Inputs, Level 2 [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	2,941.4	2,947.4
Homebuilding [Member] \| Fair Value, Inputs, Level 2 [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	0.0	0.0
Homebuilding [Member] \| 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
Homebuilding [Member] \| Fair Value, Inputs, Level 3 [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	0.0	0.0
Homebuilding [Member] \| Fair Value, Inputs, Level 3 [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Fair value	14.6	11.6
Financial Services and Other [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	45.3	31.4
Restricted cash, Carrying Value	7.3
Mortgage repurchase facility	419.0	473.0
Financial Services and Other [Member] \| Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	45.3	31.4
Restricted cash, Fair value	7.3
Mortgage repurchase facility, Fair value	419.0	473.0
Financial Services and Other [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Fair value	45.3	31.4
Restricted cash, Fair value	7.3
Mortgage repurchase facility, Fair value	0.0	0.0
Financial Services and Other [Member] \| 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
Mortgage repurchase facility, Fair value	0.0	0.0
Financial Services and Other [Member] \| 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
Mortgage repurchase facility, Fair value	$ 419.0	$ 473.0