Components of total lease cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessee Disclosure [Abstract]
Total lease cost	$ 49,938	$ 107,308	$ 184,897	$ 422,374
Operating lease cost	3,364	8,344	14,651	28,802
Short-term lease cost	27,787	72,874	107,989	309,876
Variable lease cost	18,787	26,090	62,257	83,696
Operating cash flows from operating leases			9,074	9,029
Investing cash flows from operating leases			6,751	20,256
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 0	$ 1,300	$ 745	$ 24,000