Amount of Gain Loss Recognized in Income Related to Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	$ (39)	$ 60	$ 135
Unrealized Gain (Loss) on Derivatives	649	(157)	(15)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	610	(97)	120
Natural gas contracts [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	(36)	66	144
Unrealized Gain (Loss) on Derivatives	114	(81)	(124)
Oil contracts [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	(3)	(6)	1
Unrealized Gain (Loss) on Derivatives	535	(76)	99
Basis contracts [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	0	0	(10)
Unrealized Gain (Loss) on Derivatives	$ 0	$ 0	$ 10