Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Oct. 05, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 202.4	$ 111.2
Effective tax rate (percent)	51.70%	35.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 108.7
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	21.00%
Deferred tax assets net of DTL	$ 260.8			$ 376.2
Business Acquisition, Percentage of Voting Interests Acquired					75.00%
Goodwill	100.0			$ 80.0
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				35.00%
Valuation allowance for deferred income taxes	$ 21.7			$ 11.2
Forestar Group [Member]
Income Tax Disclosure [Abstract]
Goodwill					$ 20.0
Income Tax Contingency [Line Items]
Deferred tax assets, gross					19.5
Valuation allowance for deferred income taxes					$ 19.2
Scenario, Forecast [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			24.50%