Basis of presentation and significant accounting policies - Other fixed assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pipeline and other gas gathering assets and other fixed assets
Depreciation, depletion and amortization	[1]	$ 277,724	$ 246,474	$ 233,944
Property and equipment, net		1,200,255	3,354,082
Other fixed assets
Pipeline and other gas gathering assets and other fixed assets
Depreciation, depletion and amortization		6,500	5,100	$ 4,400
Property and equipment, net		43,538	42,345
Computer hardware and software
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		12,148	13,495
Vehicles
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		9,266	7,802
Leasehold improvements
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		7,710	6,867
Real estate and buildings
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		7,618	4,908
Aircraft
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		4,952	4,952
Other
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		5,105	4,909
Depreciable total, net
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		46,799	42,933
Less accumulated depreciation		(18,169)	(13,820)
Property and equipment, net		28,630	29,113
Land
Pipeline and other gas gathering assets and other fixed assets
Other fixed assets, net		$ 14,908	$ 13,232
Minimum | Other
Pipeline and other gas gathering assets and other fixed assets
Useful life (in years)		3 years
Maximum | Other
Pipeline and other gas gathering assets and other fixed assets
Useful life (in years)		10 years

[1] Depletion, depreciation and amortization were based on actual costs for each segment with the exception of the allocation of depreciation of other fixed assets, which was based on the number of employees in the respective segment for the years ended December 31, 2015, 2014 and 2013.