Revenue from Contracts with Customers - Revenue by sales channel (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 64,878	$ 53,121
Direct
Disaggregation of Revenue [Line Items]
Revenues	38,710	31,014
Partner
Disaggregation of Revenue [Line Items]
Revenues	26,168	22,107
ASC 606
Disaggregation of Revenue [Line Items]
Revenues	59,203	47,879
ASC 606 | Direct
Disaggregation of Revenue [Line Items]
Revenues	34,968	27,620
ASC 606 | Partner
Disaggregation of Revenue [Line Items]
Revenues	24,235	20,259
Operating Lease Revenues
Disaggregation of Revenue [Line Items]
Revenues	5,675	5,242
Operating Lease Revenues | Direct
Disaggregation of Revenue [Line Items]
Revenues	3,742	3,394
Operating Lease Revenues | Partner
Disaggregation of Revenue [Line Items]
Revenues	$ 1,933	$ 1,848