Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Goodwill [Line Items]
Goodwill impairment	$ 0.0	$ 0.0	$ 0.0
Goodwill	163.5	163.5
Home Building [Member]
Goodwill [Line Items]
Goodwill	134.3	134.3
Home Building [Member] | East [Member]
Goodwill [Line Items]
Goodwill	26.4	26.4
Home Building [Member] | Midwest [Member]
Goodwill [Line Items]
Goodwill	49.7	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