Significant accounting policies - Recent accounting pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Sep. 30, 2016
Assets:
Accounts receivable	$ 11,914		$ 11,914		$ 6,839	$ 6,792
Prepaid expenses and other current assets	2,297		2,297		2,618	2,330
Other non-current assets	574		574		555	536
Liabilities:
Deferred revenue	4,909		4,909		5,394	5,437
Deferred revenue, net of current portion	357		357		477	520
Stockholders’ equity:
Accumulated deficit	(205,286)		(205,286)		(175,448)	(175,888)
Revenue	14,944	$ 10,591	40,816	$ 26,755
Total costs of goods sold and services	7,911	4,996	20,847	14,016
Gross profit	7,033	5,595	19,969	12,739
Selling, general and administrative expenses	13,352	8,846	38,293	23,117
Net income	(9,870)	(7,657)	(29,838)	(22,202)
Restricted cash	1,026	1,000	1,026	1,000		$ 1,000
Minority interest in preferred stock in privately held company
Stockholders’ equity:
Purchase price of investment							$ 300
Investment in a convertible notes of a privately held company
Stockholders’ equity:
Investment principle amount		200		200
Product revenue
Stockholders’ equity:
Revenue	10,737	5,962	29,059	15,907
Total costs of goods sold and services	5,513	3,277	14,217	8,995
Service and other revenue
Stockholders’ equity:
Revenue	4,207	3,017	11,757	8,699
Total costs of goods sold and services	2,398	$ 1,719	6,630	$ 5,021
Adjustments
Assets:
Accounts receivable					47
Prepaid expenses and other current assets	290		290		288
Other non-current assets	18		18		19
Liabilities:
Deferred revenue	175		175		43
Deferred revenue, net of current portion	9		9		43
Stockholders’ equity:
Accumulated deficit	(492)		(492)		$ (440)
Gross profit	(238)		(159)
Selling, general and administrative expenses	(55)		4
Net income	(183)		(163)
Adjustments | Product revenue
Stockholders’ equity:
Revenue	(332)		(381)
Adjustments | Service and other revenue
Stockholders’ equity:
Revenue	97		228
Total costs of goods sold and services	3		6
Balances without adoption of ASU 606
Assets:
Accounts receivable	11,914		11,914
Prepaid expenses and other current assets	2,007		2,007
Other non-current assets	592		592
Liabilities:
Deferred revenue	5,084		5,084
Deferred revenue, net of current portion	366		366
Stockholders’ equity:
Accumulated deficit	(205,778)		(205,778)
Gross profit	6,795		19,810
Selling, general and administrative expenses	13,297		38,297
Net income	(10,053)		(30,001)
Balances without adoption of ASU 606 | Product revenue
Stockholders’ equity:
Revenue	10,405		28,678
Balances without adoption of ASU 606 | Service and other revenue
Stockholders’ equity:
Revenue	4,304		11,985
Total costs of goods sold and services	$ 7,914		$ 20,853