Derivative Financial Instruments (Details 4) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	$ 2,377	$ 34,786
Commodity derivatives | Oil and gas revenue
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	610	2,633
Commodity derivatives | Derivatives, net
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	4,345	32,327
Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss), derivatives not designated as hedging instruments	$ (2,578)	(174)
Derivatives in cash flow hedging relationships
Derivative instruments, Location of Gain/(Loss)
Interest rate derivatives, net		194
Derivatives in cash flow hedging relationships | Interest rate contracts | Interest expense
Derivative instruments, Location of Gain/(Loss)
Amount of Gain/(Loss) reclassified from AOCI into earnings		$ 194