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Summary of significant accounting policies - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Accounting Policies [Abstract]          
Contract with customer, asset $ 1,600   $ 1,600   $ 900
Contract With Customer, Liability [Roll Forward]          
Balance, beginning of the period 221,579 $ 145,312 205,509 $ 140,449  
Revenue earned (66,967) (49,393) (111,398) (91,395)  
Deferral of revenue 83,845 61,363 144,346 108,228  
Balance, end of the period $ 238,457 $ 157,282 $ 238,457 $ 157,282