Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair value measurements
Cash equivalents	$ 50	$ 15
Total Assets	50	15
Total Liabilities	(32)	(65)
Commodity derivatives
Fair value measurements
Other current liabilities (commodity derivatives)	(30)	(64)
Biofuels blending obligation
Fair value measurements
Other liabilities (biofuel blending obligation)	(2)	(1)
Level 1
Fair value measurements
Cash equivalents	50	15
Total Assets	50	15
Total Liabilities	0	0
Level 1 \| Commodity derivatives
Fair value measurements
Other current liabilities (commodity derivatives)	0	0
Level 1 \| Biofuels blending obligation
Fair value measurements
Other liabilities (biofuel blending obligation)	0	0
Level 2
Fair value measurements
Cash equivalents	0	0
Total Assets	0	0
Total Liabilities	(32)	(65)
Level 2 \| Commodity derivatives
Fair value measurements
Other current liabilities (commodity derivatives)	(30)	(64)
Level 2 \| Biofuels blending obligation
Fair value measurements
Other liabilities (biofuel blending obligation)	(2)	(1)
Level 3
Fair value measurements
Cash equivalents	0	0
Total Assets	0	0
Total Liabilities	0	0
Level 3 \| Commodity derivatives
Fair value measurements
Other current liabilities (commodity derivatives)	0	0
Level 3 \| Biofuels blending obligation
Fair value measurements
Other liabilities (biofuel blending obligation)	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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- Definition

Fair value, after the effects of master netting arrangements, of a 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, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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Fair value of financial and nonfinancial obligations.

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- Definition

Fair value portion of other liabilities.

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