Amount of Gain Loss Recognized in Income Related to Derivative Financial Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	$ (32)	$ 20
Unrealized Gain (Loss) on Derivatives	(79)	33
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(111)	53
Crude oil [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	(38)	26
Unrealized Gain (Loss) on Derivatives	(76)	1
Natural gas [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Realized Gain (Loss) on Derivatives	6	(6)
Unrealized Gain (Loss) on Derivatives	$ (7)	$ 32