Basis of presentation and significant accounting policies - Other fixed assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other fixed assets
Depreciation, depletion and amortization	$ 158,389	$ 148,339	$ 277,724
Property and equipment, net	1,768,385	1,366,867
Total other fixed assets, net	40,721	44,773
Other fixed assets
Other fixed assets
Depreciation, depletion and amortization	5,900	5,900	$ 6,500
Computer hardware and software
Other fixed assets
Other fixed assets, net	11,696	12,710
Vehicles
Other fixed assets
Other fixed assets, net	9,661	7,413
Real estate and buildings
Other fixed assets
Other fixed assets, net	7,618	7,618
Leasehold improvements
Other fixed assets
Other fixed assets, net	7,590	7,549
Aircraft
Other fixed assets
Other fixed assets, net	6,402	11,352
Other
Other fixed assets
Other fixed assets, net	5,990	5,849
Depreciable total, net
Other fixed assets
Other fixed assets, net	48,957	52,491
Less accumulated depreciation and impairment	(23,150)	(22,632)
Property and equipment, net	25,807	29,859
Land
Other fixed assets
Other fixed assets, net	$ 14,914	$ 14,914
Minimum | Other fixed assets
Other fixed assets
Useful life (in years)	3 years
Maximum | Other fixed assets
Other fixed assets
Useful life (in years)	10 years