Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

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Fair Value Measurements - Carrying Value and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 1,007.8	$ 1,303.2	$ 1,383.8	$ 661.8
Notes payable, carrying value	2,871.6	3,271.3
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	973.0	1,271.8
Restricted cash, carrying value	9.3	9.5
Notes payable, carrying value	2,451.6	2,798.3
Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, carrying value	2,440.5	2,786.7
Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, carrying value	11.1	11.6
Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	34.8	31.4
Restricted cash, carrying value	7.2
Mortgage repurchase facility	420.0	473.0
Level 1 [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	973.0	1,271.8
Other Assets, Fair Value Disclosure	9.3	9.5
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	34.8	31.4
Other Assets, Fair Value Disclosure	7.2
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	2,584.1	2,947.4
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
Other Assets, Fair Value Disclosure	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	11.1	11.6
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
Other Assets, Fair Value Disclosure	0.0
Mortgage repurchase facility, fair value	420.0	473.0
Estimate fair value [Member] \| Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	973.0	1,271.8
Other Assets, Fair Value Disclosure	9.3	9.5
Estimate fair value [Member] \| Homebuilding [Member] \| Senior notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	2,584.1	2,947.4
Estimate fair value [Member] \| Homebuilding [Member] \| Secured Debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable, fair value	11.1	11.6
Estimate fair value [Member] \| Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	34.8	31.4
Other Assets, Fair Value Disclosure	7.2
Mortgage repurchase facility, fair value	$ 420.0	$ 473.0