Revenue, deferred revenue, and deferred commissions - Disaggregation of Revenue (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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 49,652	$ 50,781	$ 44,759	$ 35,309	$ 48,880	$ 47,822	$ 42,686	$ 40,242	$ 180,501	$ 179,630	$ 165,989
Product
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									92,561	97,447	91,585
Product Segment | Software [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									31,337	37,317	29,839
Product Segment | Device [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									61,224	60,130	61,746
Service Segment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									87,940	82,183	74,404
Service Segment | Maintenance and Support [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									68,846	62,267	52,342
Service Segment | Professional Services and Training [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									19,094	19,916	22,062
United States
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									164,827	161,338	148,993
International
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 15,674	$ 18,292	$ 16,996