Derivatives - Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Gross and net fair values of outstanding derivatives (in millions)
Collateral received netted against derivative assets	$ 4	$ 0
Collateral paid netted against derivative liabilities	40	54
Collateral deposited with clearinghouses and brokers	73	53
Cash-flow hedges \| Commodity Contracts \| Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Total net fair value, asset		3
Cash-flow hedges \| Level 2 \| Commodity Contracts \| Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross		3
Not designated as hedging instruments \| Commodity Contracts \| Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Total net fair value, asset	561	195
Netting, asset	(1,027)	(517)
Not designated as hedging instruments \| Commodity Contracts \| Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Total net fair value, asset	3	2
Netting, asset	(41)	(1)
Not designated as hedging instruments \| Commodity Contracts \| Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(1,027)	(517)
Total net fair value, liability	634	240
Not designated as hedging instruments \| Commodity Contracts \| Deferred credits and other liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(41)	(1)
Total net fair value, liability	2	3
Not designated as hedging instruments \| Level 1 \| Commodity Contracts \| Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	933	485
Not designated as hedging instruments \| Level 1 \| Commodity Contracts \| Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	40	1
Not designated as hedging instruments \| Level 1 \| Commodity Contracts \| Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	979	535
Not designated as hedging instruments \| Level 1 \| Commodity Contracts \| Deferred credits and other liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	40	1
Not designated as hedging instruments \| Level 2 \| Commodity Contracts \| Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	655	227
Not designated as hedging instruments \| Level 2 \| Commodity Contracts \| Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	4	2
Not designated as hedging instruments \| Level 2 \| Commodity Contracts \| Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	682	222
Not designated as hedging instruments \| Level 2 \| Commodity Contracts \| Deferred credits and other liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	$ 3	$ 3

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

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