Property and equipment - Other fixed assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization	$ 212,677	$ 158,389	$ 148,339
Total other fixed assets, net	39,819	40,721
Other fixed assets
Property, Plant and Equipment [Line Items]
Depreciation, depletion and amortization	6,100	5,900	$ 5,900
Vehicles
Property, Plant and Equipment [Line Items]
Other fixed assets, net	10,660	9,661
Computer hardware and software
Property, Plant and Equipment [Line Items]
Other fixed assets, net	9,222	11,696
Buildings
Property, Plant and Equipment [Line Items]
Other fixed assets, net	7,804	7,618
Leasehold improvements
Property, Plant and Equipment [Line Items]
Other fixed assets, net	7,608	7,590
Aircraft
Property, Plant and Equipment [Line Items]
Other fixed assets, net	6,402	6,402
Other
Property, Plant and Equipment [Line Items]
Other fixed assets, net	3,735	5,990
Depreciable total, net
Property, Plant and Equipment [Line Items]
Other fixed assets, net	45,431	48,957
Less accumulated depreciation and impairment	(23,871)	(23,150)
Total other fixed assets, net	21,560	25,807
Land
Property, Plant and Equipment [Line Items]
Total other fixed assets, net	$ 18,259	$ 14,914
Minimum | Other fixed assets
Property, Plant and Equipment [Line Items]
Useful life	3 years
Maximum | Other fixed assets
Property, Plant and Equipment [Line Items]
Useful life	10 years