Liabilities - Customer Deposits and Deferred Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Liabilities
Customer deposits	$ 45,700	$ 41,500
Movement in Deferred Revenue
Beginning Balance	70,536
Deferral of revenue	5,360
Recognition of unearned revenue	(27,100)
Ending balance	$ 48,796