Revenue, deferred revenue, and deferred commissions - Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other receivables	$ 6,312	$ 4,148
Deferred revenue, current	50,033	44,053
Deferred revenue, long-term	11,442	14,579
Total deferred revenue	61,475	58,632	$ 55,151
Stockholders' equity	$ 163,825	$ 162,867