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Fair Value Measurements - Carrying Value and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 1,303.2	$ 1,383.8	$ 661.8	$ 977.4
Notes payable, carrying value	3,271.3	3,811.5
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	1,271.8	1,354.8
Restricted cash, carrying value	9.5	9.7
Notes payable, carrying value	2,798.3	3,333.6
Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, carrying value	2,786.7	3,325.5
Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, carrying value	11.6	8.1
Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	31.4	29.0
Mortgage repurchase facility	473.0	477.9
Level 1 [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	1,271.8	1,354.8
Other Assets, Fair Value Disclosure	9.5	9.7
Level 1 [Member] \| Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	0.0	0.0
Level 1 [Member] \| Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	0.0	0.0
Level 1 [Member] \| Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	31.4	29.0
Mortgage repurchase facility, fair value	0.0	0.0
Level 2 [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Level 2 [Member] \| Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	2,947.4	3,405.9
Level 2 [Member] \| Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	0.0	0.0
Level 2 [Member] \| Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Mortgage repurchase facility, fair value	0.0	0.0
Level 3 [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Level 3 [Member] \| Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	0.0	0.0
Level 3 [Member] \| Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	11.6	8.1
Level 3 [Member] \| Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Mortgage repurchase facility, fair value	473.0	477.9
Estimate fair value [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	1,271.8	1,354.8
Other Assets, Fair Value Disclosure	9.5	9.7
Estimate fair value [Member] \| Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	2,947.4	3,405.9
Estimate fair value [Member] \| Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	11.6	8.1
Estimate fair value [Member] \| Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	31.4	29.0
Mortgage repurchase facility, fair value	$ 473.0	$ 477.9