Revenue, deferred revenue, and deferred commissions - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 47,347	$ 44,759	$ 88,020	$ 80,068
Product Segment [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	23,951	23,132	41,801	37,135
Product Segment [Member] | Software [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	6,851	8,628	10,798	12,571
Product Segment [Member] | Device [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	17,100	14,504	31,003	24,564
Service Segment [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	23,396	21,627	46,219	42,933
Service Segment [Member] | Maintenance and Support [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	18,994	16,928	37,063	33,321
Service Segment [Member] | Professional Services and Training [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 4,402	$ 4,699	$ 9,156	$ 9,612