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Components of total lease cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Lessee Disclosure [Abstract]    
Total lease cost $ 232,730 $ 442,949
Operating lease cost 17,980 35,570
Short-term lease cost 143,892 301,373
Variable lease cost 70,858 106,006
Operating cash flows from operating leases 12,046 12,074
Investing cash flows from operating leases 7,313 24,129
Right-of-use assets obtained in exchange for new operating lease liabilities $ 745 $ 25,400