Derivative Financial Instruments Gains and Losses (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		$ (101,710)	$ (112,795)	$ (248,738)	$ (274,024)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		163,351	80,929	149,123	(73,238)
NYMEX Oil Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		(72,164)	(73,915)	(172,156)	(180,129)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		60,773	66,749	50,341	(7,111)
Gas Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Gain on Early Settlement of Derivatives			(15,300)
Derivative settlement gain (loss)	[1]	(31,439)	(38,880)	(72,492)	(73,112)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		62,489	6,070	38,466	(76,269)
NGL Contracts [Member]
Gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Derivative settlement gain (loss)		1,893	0	(4,090)	(20,783)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		$ 40,089	$ 8,110	$ 60,316	$ 10,142

[1] Natural gas derivative settlements for the three and six months ended June 30, 2015, include a $15.3 million gain on the early settlement of future contracts as a result of divesting our Mid-Continent assets during the second quarter of 2015.