Segment Information and Information about Geographic Area - Revenue by geographic area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 64,878	$ 53,121
United States
Disaggregation of Revenue [Line Items]
Revenues	44,314	37,461
Foreign
Disaggregation of Revenue [Line Items]
Revenues	20,564	15,660
ASC 606
Disaggregation of Revenue [Line Items]
Revenues	59,203	47,879
ASC 606 | United States
Disaggregation of Revenue [Line Items]
Revenues	40,308	33,882
ASC 606 | Foreign
Disaggregation of Revenue [Line Items]
Revenues	18,895	13,997
Operating Lease Revenues
Disaggregation of Revenue [Line Items]
Revenues	5,675	5,242
Operating Lease Revenues | United States
Disaggregation of Revenue [Line Items]
Revenues	4,006	3,579
Operating Lease Revenues | Foreign
Disaggregation of Revenue [Line Items]
Revenues	$ 1,669	$ 1,663