Derivative Instruments and Hedging Activities - Fair Value Amounts and (Gain) Loss on Commodity Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 29		$ 29		$ 2
Derivative Liability, Fair Value	335		335		73
Total Cash Paid (Received) in Settlement of Commodity Derivative Instruments	65	$ (11)	93	$ (14)
Total Non-cash Portion of Loss (Gain) on Commodity Derivative Instruments	184	(46)	235	(153)
Total Loss (Gain) on Commodity Derivative Instruments	249	(57)	328	(167)
Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	29		29		2
Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	250		250		58
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	0		0		0
Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	85		85		$ 15
Crude Oil
Derivatives, Fair Value [Line Items]
Total Cash Paid (Received) in Settlement of Commodity Derivative Instruments	66	(11)	96	(16)
Total Non-cash Portion of Loss (Gain) on Commodity Derivative Instruments	181	(28)	231	(91)
Total Loss (Gain) on Commodity Derivative Instruments	247	(39)	327	(107)
Natural Gas
Derivatives, Fair Value [Line Items]
Total Cash Paid (Received) in Settlement of Commodity Derivative Instruments	(1)	0	(3)	2
Total Non-cash Portion of Loss (Gain) on Commodity Derivative Instruments	3	(18)	4	(62)
Total Loss (Gain) on Commodity Derivative Instruments	$ 2	$ (18)	$ 1	$ (60)

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

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