Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Oct. 05, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 94.0		$ 124.7	$ 296.4	$ 236.0
Effective tax rate (percent)	21.10%		35.20%	35.50%	35.10%
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	$ 245.8			$ 245.8			$ 376.2
Business Acquisition, Percentage of Voting Interests Acquired								75.00%
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	25.9			25.9			$ 11.2
Goodwill	$ 109.2			$ 109.2			$ 80.0
Forestar Group [Member]
Income Tax Contingency [Line Items]
Deferred tax assets, gross								$ 20.4
Valuation allowance for deferred income taxes								20.1
Goodwill								$ 29.2
Scenario, Forecast [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent						24.50%