Derivatives - Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Gross and net fair values of outstanding derivatives (in millions)
Collateral received netted against derivative assets	$ 0	$ 0
Collateral paid netted against derivative liabilities	32	54
Collateral deposited with clearinghouses and brokers	71	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	149	195
Netting, asset	(613)	(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	2	2
Netting, asset	(7)	(1)
Not designated as hedging instruments | Commodity Contracts | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(613)	(517)
Total net fair value, liability	168	240
Not designated as hedging instruments | Commodity Contracts | Deferred credits and liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(7)	(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	581	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	7	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	613	535
Not designated as hedging instruments | Level 1 | Commodity Contracts | Deferred credits and liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	6	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	181	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	2	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	168	222
Not designated as hedging instruments | Level 2 | Commodity Contracts | Deferred credits and liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	$ 3	$ 3