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.

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Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Line Items]
Goodwill impairment	$ 0.0	$ 0.0	$ 0.0
Goodwill	163.5	163.5
Home Building
Goodwill [Line Items]
Goodwill	134.3	134.3
Home Building \| Northwest
Goodwill [Line Items]
Goodwill	2.2	2.2
Home Building \| Southwest [Member]
Goodwill [Line Items]
Goodwill	0.0	0.0
Home Building \| South Central [Member]
Goodwill [Line Items]
Goodwill	15.9	15.9
Home Building \| Southeast [Member]
Goodwill [Line Items]
Goodwill	6.0	6.0
Home Building \| East [Member]
Goodwill [Line Items]
Goodwill	60.5	60.5
Home Building \| North
Goodwill [Line Items]
Goodwill	49.7	49.7
Intersegment Eliminations [Member]
Goodwill [Line Items]
Goodwill	$ 29.2	$ 29.2