Segment Information (Long-lived assets by Geographic Location) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	$ 185,440	$ 183,514
United States
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	138,912	136,203
Europe
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	28,449	28,261
Other Geographic Locations
Segment Reporting Information [Line Items]
Property and equipment, net of accumulated depreciation	$ 18,079	$ 19,050