Summary of significant accounting policies - Deferred Contract Costs (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Concentration Risk [Line Items]
Total amortization of contract costs	$ 3,100,000	$ 2,000,000	$ 9,000,000	$ 5,500,000
Impairment losses	0	$ 0	0	$ 0
Trade accounts receivable, net of allowances	69,486,000		69,486,000		$ 69,056,000
Accounts Receivable | Credit Concentration Risk
Concentration Risk [Line Items]
Trade accounts receivable, net of allowances	$ 10,800,000		$ 10,800,000
Accounts Receivable | Credit Concentration Risk | Two Distributors
Concentration Risk [Line Items]
Trade accounts receivable, net of allowances					$ 19,800,000