Liabilities - Customer Deposits and Deferred Revenue (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Liabilities
Customer deposits	$ 20,000	$ 41,500
Movement in Deferred Revenue
Beginning balance	70,536
Deferral of revenue	7,166
Recognition of previously deferred revenue	(33,224)
Ending balance	$ 44,478