Significant accounting policies - Recent accounting pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Assets:
Accounts receivable	$ 10,906				$ 6,792				$ 10,906	$ 6,792		$ 6,839
Prepaid expenses and other current assets	2,137				2,330				2,137	2,330		2,618
Other non-current assets	557				536				557	536		555
Liabilities:
Deferred revenue	4,697				5,437				4,697	5,437		5,394
Deferred revenue, net of current portion	466				520				466	520		477
Stockholders’ equity:
Accumulated deficit	(216,244)				(175,888)				(216,244)	(175,888)		(175,448)
Revenue	15,917	$ 14,944	$ 13,536	$ 12,337	10,877	$ 10,591	$ 8,643	$ 7,521	56,734	37,632	$ 22,874
Total costs of goods sold and services	9,052	7,911	6,605	6,330	5,669	4,996	4,670	4,349	29,898	19,684	12,887
Gross profit	6,865	7,033	6,931	6,007	5,208	5,595	3,973	3,172	26,836	17,948	9,987
Selling, general and administrative	13,953	13,352	13,429	11,512	10,577	8,846	7,579	6,691	52,246	33,693	19,688
Net income	(10,958)	(9,870)	(10,563)	(9,405)	(9,334)	(7,657)	(7,343)	(7,202)	(40,796)	(31,536)	(27,019)
Restricted cash	1,026				1,000				1,026	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	11,431	10,737	8,776	9,547	7,458	5,962	5,200	4,745	40,491	23,365	14,124
Total costs of goods sold and services	6,684	5,513	4,455	4,248	3,734	3,277	2,945	2,773	20,900	12,729	7,742
Service and other revenue
Stockholders’ equity:
Revenue	4,302	4,207	4,760	2,790	3,419	3,017	3,174	2,507	16,059	12,117	7,676
Total costs of goods sold and services	2,368	$ 2,398	$ 2,150	$ 2,082	1,935	$ 1,719	$ 1,725	$ 1,576	8,998	6,955	$ 5,145
Adjustments
Assets:
Accounts receivable					47					47
Prepaid expenses and other current assets	335				288				335	288
Other non-current assets					19					19
Liabilities:
Deferred revenue	(209)				43				(209)	43
Deferred revenue, net of current portion	14				43				14	43
Stockholders’ equity:
Accumulated deficit	(140)				$ (440)				(140)	$ (440)		$ 400
Total costs of goods sold and services									1
Gross profit									327
Selling, general and administrative									27
Net income									300
Adjustments | Product revenue
Stockholders’ equity:
Revenue									55
Adjustments | Service and other revenue
Stockholders’ equity:
Revenue									273
Balances without adoption of ASU 606
Assets:
Accounts receivable	10,906								10,906
Prepaid expenses and other current assets	1,802								1,802
Other non-current assets	557								557
Liabilities:
Deferred revenue	4,488								4,488
Deferred revenue, net of current portion	480								480
Stockholders’ equity:
Accumulated deficit	$ (216,104)								(216,104)
Total costs of goods sold and services									29,899
Gross profit									27,163
Selling, general and administrative									52,273
Net income									(40,496)
Balances without adoption of ASU 606 | Product revenue
Stockholders’ equity:
Revenue									40,546
Balances without adoption of ASU 606 | Service and other revenue
Stockholders’ equity:
Revenue									$ 16,332