Derivative and Hedging Activities - Derivative Instruments in Balance Sheet Table (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (338)	$ (35)
Not designated as hedging instrument [Member] \| Commodity contract [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	(338)	(35)
Derivative liability, fair value, gross asset	0	0
Derivative, fair value, net	(338)	(35)
Other current assets [Member] \| Not designated as hedging instrument [Member] \| Commodity contract [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	14	157
Derivative asset, fair value, gross liability	(14)	(130)
Derivative, fair value, net	0	27
Other noncurrent assets [Member] \| Not designated as hedging instrument [Member] \| Commodity contract [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	3
Derivative asset, fair value, gross liability	(3)
Derivative, fair value, net	0
Other current liabilities [Member] \| Not designated as hedging instrument [Member] \| Commodity contract [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	(324)	(188)
Derivative liability, fair value, gross asset	14	130
Derivative, fair value, net	(310)	(58)
Other noncurrent liabilities [Member] \| Not designated as hedging instrument [Member] \| Commodity contract [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value, gross liability	(31)	(4)
Derivative liability, fair value, gross asset	3	0
Derivative, fair value, net	$ (28)	$ (4)

Fair value of liability associated with 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.

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Fair value of asset associated with financial liability or 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.

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