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Summary of significant accounting policies - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Change in Contract with Customer, Liability [Abstract]        
Balance, beginning of the period $ 145,735 $ 111,255 $ 140,710 $ 100,662
Revenue earned (49,562) (42,277) (97,285) (76,884)
Deferral of revenue 61,565 48,941 114,313 94,141
Balance, end of the period $ 157,738 $ 117,919 $ 157,738 $ 117,919