Amount of Gain Loss Recognized in Income Related to Derivative Financial Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	$ 201	$ 505	$ (39)
Unrealized Gain (Loss) on Derivatives	(392)	(246)	649
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(191)	259	610
Natural gas contracts [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	2	130	(36)
Unrealized Gain (Loss) on Derivatives	(76)	(81)	114
Oil contracts [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	199	375	(3)
Unrealized Gain (Loss) on Derivatives	$ (316)	$ (165)	$ 535