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Employee benefit plans - Allocation of Stock-based Compensation Expense (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]      
Total stock-based compensation expense $ 29,963,000 $ 37,188,000 $ 40,887,000
Share-based Payment Arrangement, Expense, Tax Benefit 0 0 0
Cost of Revenue [Member]      
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]      
Total stock-based compensation expense 1,548,000 1,902,000 1,954,000
Research and Development [Member]      
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]      
Total stock-based compensation expense 13,415,000 17,167,000 19,636,000
Selling and Marketing Expense [Member]      
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]      
Total stock-based compensation expense 5,779,000 8,043,000 9,459,000
General and Administrative [Member]      
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]      
Total stock-based compensation expense $ 9,221,000 $ 10,076,000 $ 9,838,000