Property and Equipment, Field Equipment and Deferred Cost of Revenues - Components of Assets Subject to Capital Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Less accumulated depreciation	$ (8,979)	$ (6,402)
Assets subject to capital leases, net	19,609	17,629
Manufacturing facility in Germany | Germany
Property, Plant and Equipment [Line Items]
Manufacturing facility in Germany/Other assets subject to capital leases	24,135	21,007
Other assets subject to capital lease
Property, Plant and Equipment [Line Items]
Manufacturing facility in Germany/Other assets subject to capital leases	$ 4,453	$ 3,024