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Fair Value Measurements - Assets and Liabilities Not Reflected at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	$ 512.4	$ 1,303.2	$ 906.8	$ 1,383.8
Notes payable, Carrying value	2,926.5	3,271.3
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	460.8	1,271.8
Restricted cash, Carrying value	10.9	9.5
Notes payable, Carrying value	2,453.1	2,798.3
Homebuilding [Member] \| Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Carrying value	2,439.7	2,786.7
Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, Carrying value	13.4	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	460.8	1,271.8
Restricted cash, Fair value	10.9	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	2,583.3	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	13.4	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	460.8	1,271.8
Restricted cash, Fair value	10.9	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	2,583.3	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	13.4	11.6
Financial Services and Other [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying value	51.6	31.4
Restricted cash, Carrying Value	8.5
Mortgage repurchase facility	473.4	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	51.6	31.4
Restricted cash, Fair value	8.5
Mortgage repurchase facility, Fair value	473.4	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	51.6	31.4
Restricted cash, Fair value	8.5
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	$ 473.4	$ 473.0