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Summary of significant accounting policies - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Change in Contract with Customer, Liability [Abstract]        
Balance, beginning of the period $ 157,738 $ 117,919 $ 140,710 $ 100,662
Revenue earned (50,038) (39,261) (147,324) (116,145)
Deferral of revenue 80,538 54,651 194,852 148,792
Balance, end of the period $ 188,238 $ 133,309 $ 188,238 $ 133,309