Derivatives - Not Designated as Hedging Instruments (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) $ / MillionCubicFeet $ / Barrel Bcf MMBbls	Dec. 31, 2017 USD ($) $ / MillionCubicFeet $ / Barrel Bcf MMBbls
Outstanding commodity derivatives contracts not designated as hedging instruments
Derivative instrument settlement period (within)	3 months
Not designated as hedging instruments \| Crude oil
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price \| $ / Barrel	69.98	57.38
Not designated as hedging instruments \| Crude oil \| Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges	$ (37)	$ (47)
Not designated as hedging instruments \| Natural gas
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price \| $ / MillionCubicFeet	2.67	2.73
Not designated as hedging instruments \| Natural gas \| Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges	$ (1)	$ 1
Not designated as hedging instruments \| Long position \| Crude oil
Outstanding commodity derivatives contracts not designated as hedging instruments
Outstanding net volumes on derivatives not designated as hedges (mmbls/bcf) \| MMBbls	73	61
Not designated as hedging instruments \| Short position \| Natural gas
Outstanding commodity derivatives contracts not designated as hedging instruments
Outstanding net volumes on derivatives not designated as hedges (mmbls/bcf) \| Bcf	88	47

The revenues derived from sales of oil or gas in each geographic area during the reporting period divided by the number of barrels of oil produced or thousands of cubic feet of gas produced.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1204
-Paragraph (b)
-Subparagraph (1)

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998890-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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