Derivatives - Fair Value (Details) - USD ($)	Sep. 30, 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	64,000,000	54,000,000
Collateral deposited with clearinghouses and brokers	73,000,000	53,000,000
Cash-flow hedges | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Netting, asset		0
Total net fair value, asset		3,000,000
Cash-flow hedges | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	0
Total net fair value, liability	1,000,000
Cash-flow hedges | Level 1 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross		0
Cash-flow hedges | Level 1 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	0
Cash-flow hedges | Level 2 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross		3,000,000
Cash-flow hedges | Level 2 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	1,000,000
Cash-flow hedges | Level 3 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross		0
Cash-flow hedges | Level 3 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	0
Not designated as hedging instruments | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Netting, asset	1,235,000,000	517,000,000
Total net fair value, asset	606,000,000	195,000,000
Not designated as hedging instruments | Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Netting, asset	101,000,000	1,000,000
Total net fair value, asset	4,000,000	2,000,000
Not designated as hedging instruments | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(1,235,000,000)	(517,000,000)
Total net fair value, liability	644,000,000	240,000,000
Not designated as hedging instruments | Deferred credits and other liabilities - other
Gross and net fair values of outstanding derivatives (in millions)
Netting and collateral, liability	(101,000,000)	(1,000,000)
Total net fair value, liability	4,000,000	3,000,000
Not designated as hedging instruments | Level 1 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	1,210,000,000	485,000,000
Not designated as hedging instruments | Level 1 | Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	101,000,000	1,000,000
Not designated as hedging instruments | Level 1 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	1,252,000,000	535,000,000
Not designated as hedging instruments | Level 1 | Deferred credits and other liabilities - other
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	101,000,000	1,000,000
Not designated as hedging instruments | Level 2 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	631,000,000	227,000,000
Not designated as hedging instruments | Level 2 | Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	4,000,000	2,000,000
Not designated as hedging instruments | Level 2 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	627,000,000	222,000,000
Not designated as hedging instruments | Level 2 | Deferred credits and other liabilities - other
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	4,000,000	3,000,000
Not designated as hedging instruments | Level 3 | Other current assets
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	0	0
Not designated as hedging instruments | Level 3 | Long-term receivables and other assets, net
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative asset, gross	0	0
Not designated as hedging instruments | Level 3 | Accrued liabilities
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	0	0
Not designated as hedging instruments | Level 3 | Deferred credits and other liabilities - other
Gross and net fair values of outstanding derivatives (in millions)
Commodity contract derivative liability, gross	$ 0	$ 0