Clayton Williams Energy Acquisition (Details) $ / shares in Units, shares in Thousands, a in Thousands, $ in Millions		2 Months Ended	3 Months Ended			6 Months Ended
	Apr. 24, 2017 USD ($) a $ / shares shares	Jun. 30, 2017 USD ($) MMBoe	Jun. 30, 2017 USD ($) MMBoe	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) a MMBoe shares	Jun. 30, 2016 USD ($)
Business Acquisition [Line Items]
Proved undeveloped reserves (MMBOEs) | MMBoe		551	551			551
Cash consideration						$ 616	$ 0
Consideration				$ 301
Stock price ($ per share) | $ / shares	$ 34.17
Business Combination, Acquisition Related Costs			$ 90		$ 0	$ 94	$ 0
Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a	118
Increase (MMBOEs) | MMBoe						86
Proved developed reserves (MMBOEs) | MMBoe		17	17			17
Proved undeveloped reserves (MMBOEs) | MMBoe		69	69			69
Shares exchange in acquisition (shares) | shares	56,000
Equity consideration	$ 1,876
Cash consideration	637
Consideration	2,513
Credit facility draw	1,300
Merger expenses						$ 94
Business Combination, Acquisition Related Costs			$ 26			60
Noncash share compensation						$ 34
Treasury shares received in acquisition | shares						720
Increase to treasury stock						$ 25
Debt assumed	$ 595
Pro forma revenue		$ 25
Clayton Williams Energy
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs			$ 90			$ 94
Delaware Basin | Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a	71
Permian | Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a	100
TEXAS | Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a						64