Basis of presentation and description of business - Basis of Presentation and Revision Narrative (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended			12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020		Sep. 30, 2021	Sep. 30, 2020		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Revenue	$ 95,621		$ 70,548	[1]	$ 262,586	$ 192,865	[1]
Sales and marketing	40,856		23,773	[1]	103,640	67,558	[1]
Subscription
Business Acquisition [Line Items]
Revenue	$ 90,700		65,634	[1]	$ 245,900	178,438	[1]
Adjustments | Error in Commissions Capitalized
Business Acquisition [Line Items]
Sales and marketing		$ 800						$ 2,500	$ 2,000	$ 1,800
Deferred contract costs		$ (800)						$ (2,500)	$ (2,000)	$ (1,800)
Adjustments | Subscription | Classification of on Premise Subscription Revenue
Business Acquisition [Line Items]
Revenue			$ 7,800			$ 18,200

[1] Certain prior period amounts have been revised to correct immaterial errors. See Note 1 for more information.