Industry and Geographic Information - Long-lived Assets by Geographic Territory (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Long-lived assets (excluding intangible assets)	$ 5,443	$ 4,890
Property and equipment | Geographic concentration
Segment Reporting Information [Line Items]
Long-lived assets (excluding intangible assets)	5,443	4,890
Property and equipment | Geographic concentration | Domestic
Segment Reporting Information [Line Items]
Long-lived assets (excluding intangible assets)	4,210	3,779
Property and equipment | Geographic concentration | Foreign
Segment Reporting Information [Line Items]
Long-lived assets (excluding intangible assets)	$ 1,233	$ 1,111