Clayton Williams Energy Acquisition (Details) $ / shares in Units, shares in Thousands, a in Thousands, $ in Millions		3 Months Ended			5 Months Ended	6 Months Ended	9 Months Ended
	Apr. 24, 2017 USD ($) a $ / shares shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) MMBoe	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) MMBoe	Sep. 30, 2017 USD ($) a shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Proved undeveloped reserves (MMBOEs) | MMBoe			551			551
Cash consideration							$ 616	$ 0
Acquisition related costs		$ 4		$ 0			98	$ 0
Goodwill		1,295			$ 1,295		1,295		$ 0
Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a	118
Shares exchange in acquisition (shares) | shares	56,000
Equity consideration	$ 1,876
Cash consideration	637
Consideration	$ 2,513
Stock price ($ per share) | $ / shares	$ 34.17
Credit facility draw			$ 1,300			$ 1,300
Acquisition related costs			$ 23				64
Noncash share compensation							$ 34
Treasury shares received in acquisition | shares							720
Increase to treasury stock							$ 25
Debt assumed	$ 595
Goodwill	$ 1,295	1,300			1,300		1,300
Pro forma revenue					56
Pro forma loss					$ 14
Clayton Williams Energy
Business Acquisition [Line Items]
Acquisition related costs		$ 4					$ 98
Tax rate		35.00%
Delaware Basin | Clayton Williams Energy
Business Acquisition [Line Items]
Area acquired (acres) | a	71
Increase (MMBOEs) | MMBoe						86
Proved developed reserves (MMBOEs) | MMBoe			17			17
Proved undeveloped reserves (MMBOEs) | MMBoe			69			69
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