Capitalized Interest (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Statement [Line Items]
Real Estate Inventory, Capitalized Interest Costs	$ 3.2		$ 48.7	$ 23.7	$ 0.5
Interest Costs Incurred	7.3		43.3	25.3
Interest expense	(3.7)	$ (6.4)	0.0	0.0
Real Estate Inventory, Capitalized Interest Costs, Cost of Sales	$ (0.9)		$ (18.3)	$ (4.8)