Derivative Instruments and Hedging Activities - Fair Value Amounts and (Gain) Loss on Commodity Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 115		$ 115		$ 180
Derivative Liability, Fair Value	13		13		27
Total Cash (Received) Paid in Settlement of Commodity Derivative Instruments	(13)	$ 67	(28)	$ 160
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	(116)	88	51	323
Total (Gain) Loss on Commodity Derivative Instruments	(129)	155	23	483
Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	90		90		180
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	10		10		1
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	25		25		0
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	3		3		$ 26
Crude Oil
Derivatives, Fair Value [Line Items]
Total Cash (Received) Paid in Settlement of Commodity Derivative Instruments	(6)	68	(8)	164
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	(115)	85	54	316
Total (Gain) Loss on Commodity Derivative Instruments	(121)	153	46	480
Natural Gas
Derivatives, Fair Value [Line Items]
Total Cash (Received) Paid in Settlement of Commodity Derivative Instruments	(7)	(1)	(20)	(4)
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	(1)	3	(3)	7
Total (Gain) Loss on Commodity Derivative Instruments	$ (8)	$ 2	$ (23)	$ 3

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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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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