Summary of Significant Accounting Policies Summary of SIgnificant Accounting Policies - Capitalized Interest (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Dec. 31, 2017
Interest Costs Incurred [Abstract]
Interest Costs Incurred	$ 7.3		$ 25.3
Interest expense	(3.7)	$ (6.4)	0.0	$ (8.5)
Real Estate Inventory, Capitalized Interest Costs	3.2		23.7	$ 0.5
Real Estate Inventory, Capitalized Interest Costs, Cost of Sales	$ (0.9)		$ (4.8)