Revenue, deferred revenue, and deferred commissions - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Payment terms on invoiced amounts	30 days
Contract assets	$ 4,300	$ 2,400
Deferred commissions	$ 10,477	10,303	$ 10,301
Percentage of deferred commissions to be recognized as commission expense in the next 12 months	49.00%
Revenue recognized pertaining to amounts deferred as of December 31, 2017	$ 48,600	$ 42,600
Deferred revenue and backlog	$ 123,500
Percentage of deferred revenue to be recognized over the next 12 months	66.00%