Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 05, 2017
Goodwill [Line Items]
Goodwill impairment	$ 0.0	$ 0.0	$ 7.2
Goodwill	109.2	80.0
Homebuilding [Member]
Goodwill [Line Items]
Goodwill impairment			$ 7.2
Goodwill	80.0	80.0
Homebuilding [Member] | East [Member]
Goodwill [Line Items]
Goodwill	21.8	21.8
Homebuilding [Member] | Midwest [Member]
Goodwill [Line Items]
Goodwill	0.0	0.0
Homebuilding [Member] | Southeast [Member]
Goodwill [Line Items]
Goodwill	40.1	40.1
Homebuilding [Member] | South Central [Member]
Goodwill [Line Items]
Goodwill	15.9	15.9
Homebuilding [Member] | Southwest [Member]
Goodwill [Line Items]
Goodwill	0.0	0.0
Homebuilding [Member] | West [Member]
Goodwill [Line Items]
Goodwill	2.2	2.2
Forestar Consolidated [Member]
Goodwill [Line Items]
Goodwill		$ 0.0		$ 29.2
Segment Reconciling Items [Member]
Goodwill [Line Items]
Goodwill	$ 29.2