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Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018
Goodwill [Line Items]
Goodwill impairment	$ 0.0	$ 0.0	$ 0.0
Goodwill	163.5	109.2
Home Building [Member]
Goodwill [Line Items]
Goodwill	134.3	80.0
Home Building [Member] | East [Member]
Goodwill [Line Items]
Goodwill	26.4	21.8		$ 4.6
Home Building [Member] | Midwest [Member]
Goodwill [Line Items]
Goodwill	49.7	0.0		$ 49.7
Home Building [Member] | Southeast [Member]
Goodwill [Line Items]
Goodwill	40.1	40.1
Home Building [Member] | South Central [Member]
Goodwill [Line Items]
Goodwill	15.9	15.9
Home Building [Member] | Southwest [Member]
Goodwill [Line Items]
Goodwill	0.0	0.0
Home Building [Member] | West [Member]
Goodwill [Line Items]
Goodwill	2.2	2.2
Segment Reconciling Items [Member]
Goodwill [Line Items]
Goodwill	$ 29.2	$ 29.2

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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