Income Taxes - Textual (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 05, 2017
Operating Loss Carryforwards [Line Items]
Effective tax rate		29.00%	35.20%	34.50%
Federal statutory rate			35.00%	35.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		21.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 108.7	$ 108.7	$ 0.0	$ 0.0
Business Acquisition, Percentage of Voting Interests Acquired					75.00%
Goodwill		109.2	80.0
Valuation allowance		17.7	11.2
Deferred Tax Assets, Gross		299.2	427.1
Net operating loss carryforwards		38.1	26.2
Forestar Consolidated [Member]
Operating Loss Carryforwards [Line Items]
Goodwill			0.0		$ 29.2
Valuation allowance		3.5			20.1
Deferred Tax Assets, Gross					$ 20.4
Forestar Consolidated [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		4.0
Forestar Consolidated [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		14.8
Homebuilding [Member]
Operating Loss Carryforwards [Line Items]
Goodwill		80.0	$ 80.0
Valuation allowance		14.2
Homebuilding [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		19.3
Homebuilding [Member] | State [Member] | NOL Carryforwards to Expire in One to Ten Years [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration		5.4
Homebuilding [Member] | State [Member] | NOL Carryforward to Expire in Eleven to Twenty Years [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration		$ 13.9
Weighted Average [Member]
Operating Loss Carryforwards [Line Items]
Federal statutory rate		24.50%
Forestar Group [Member]
Operating Loss Carryforwards [Line Items]
Business Acquisition, Date of Acquisition Agreement	Oct. 05, 2017