Commitments and Contingencies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) lot	Dec. 31, 2016 USD ($) lot	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Letters of credit outstanding	$ 0.2	$ 0.0
Lots under purchase options | lot	1,724	954
Selling, General and Administrative Expenses
Operating Leased Assets [Line Items]
Rent expense	$ 0.8	$ 0.7	$ 0.6