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Fair Value Measurements - Carrying Value and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, carrying value	$ 1,473.1	$ 1,007.8	$ 1,303.2	$ 1,383.8
Restricted cash, carrying value	32.9	16.5
Notes payable, carrying value	3,203.5	2,871.6
Homebuilding [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, carrying value	1,111.8	973.0
Restricted cash, carrying value	8.6	9.3
Notes payable, carrying value	2,445.9	2,451.6
Financial Services [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, carrying value	33.7	24.1
Restricted cash, carrying value	8.1	7.2
Notes payable, carrying value	637.7	420.0
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	1,473.1	1,007.8
Restricted cash, fair value	32.9	16.5
Notes payable, fair value	0.0	0.0
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	2,602.6	2,584.1
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	642.2	431.1
Estimate fair value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, fair value	1,473.1	1,007.8
Restricted cash, fair value	32.9	16.5
Notes payable, fair value	$ 3,244.8	$ 3,015.2