Commitments and Contingencies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) lot	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($) lot
Operating Leased Assets [Line Items]
Letters of credit outstanding	$ 0.2			$ 2.2
Lots under purchase options | lot	1,724			1,843
Selling, General and Administrative Expenses
Operating Leased Assets [Line Items]
Rent expense	$ 1.2	$ 0.9	$ 0.7