Inventory Other Inventory Items - Homes under construction not subject to a sale contract (Details) $ in Millions	Dec. 31, 2017 USD ($) homes	Dec. 31, 2016 USD ($) homes
Other Inventory, Gross [Abstract]
Number of Speculative Homes | homes	1,134	996
Speculative Homes Carrying Value | $	$ 242.7	$ 199.4