Significant accounting policies - Segment information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenue	$ 10,877	$ 10,591	$ 8,643	$ 7,521	$ 6,589	$ 5,734	$ 5,213	$ 5,338	$ 37,632	$ 22,874	$ 17,585
Revenue by region as a percentage of total revenue									100.00%	100.00%	100.00%
North America
Disaggregation of Revenue [Line Items]
Total revenue									$ 22,706	$ 13,864	$ 13,018
Revenue by region as a percentage of total revenue									60.00%	61.00%	74.00%
EMEA
Disaggregation of Revenue [Line Items]
Total revenue									$ 11,742	$ 6,922	$ 3,416
Revenue by region as a percentage of total revenue									31.00%	30.00%	19.00%
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue									$ 3,184	$ 2,088	$ 1,151
Revenue by region as a percentage of total revenue									8.00%	9.00%	7.00%
Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	7,458	5,962	5,200	4,745	4,069	3,293	3,337	3,425	$ 23,365	$ 14,124	$ 10,601
Service and other revenue
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,419	3,017	3,174	2,507	2,252	2,172	1,608	1,644	12,117	7,676	5,012
Collaboration and license revenue
Disaggregation of Revenue [Line Items]
Total revenue		$ 1,612	$ 269	$ 269	$ 268	$ 269	$ 268	$ 269	$ 2,150	$ 1,074	$ 1,972