Income Taxes - Textual (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Effective tax rate	20.20%	23.80%	29.00%
Federal statutory rate	21.00%	21.00%	35.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 0.0	$ 0.0	$ 108.7
Deferred Tax Assets, Valuation Allowance	7.5	18.7
Net operating loss carryforwards	17.5	31.5
Effective Income Tax Rate Reconciliation, Tax Credit, Amount	93.4	$ 1.6	$ 19.0
Forestar Consolidated [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	1.7
Home Building [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	15.8
Home Building [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
Home Building [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	$ 10.4
Weighted Average [Member]
Operating Loss Carryforwards [Line Items]
Federal statutory rate			24.50%