Clayton Williams Energy Acquisition - Narrative (Details) $ / shares in Units, a in Thousands, $ in Millions		5 Months Ended	8 Months Ended	12 Months Ended
	Apr. 24, 2017 USD ($) a $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) a shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Jun. 30, 2017 USD ($)	Dec. 30, 2015 USD ($)
Business Acquisition [Line Items]
Stock issued (shares) | shares				54,087,136	0
Cash paid				$ 616	$ 0	$ 0
Clayton Williams Acquisition Expenses				100	0	$ 0
Goodwill			$ 1,310	1,310	0			$ 779
Revenue since acquisition		$ 181			457
Pre-tax loss since acquisition		$ (120)			$ (20)
Clayton Williams Energy
Business Acquisition [Line Items]
Clayton Williams Acquisition Expenses				23
Clayton Williams Energy
Business Acquisition [Line Items]
Stock issued (shares) | shares	56,000,000
Fair value of common stock issued	$ 1,876
Cash paid	637
Consideration transferred	$ 2,513
Share price ($ per share) | $ / shares	$ 34.17
Long-term line of credit							$ 1,300
Clayton Williams Acquisition Expenses				100
Severance, consulting, investment, advisory, legal, and other merger related fees				64
Noncash share-based compensation expense				$ 36
Treasury stock redeemed (in shares) | shares				720,000
Consideration transferred, treasury stock				$ 25
Long-term debt acquired	$ 595
Goodwill	$ 1,310		1,300	$ 1,300
Revenue since acquisition			99
Pre-tax loss since acquisition			$ 19
Clayton Williams Energy | Delaware Basin
Business Acquisition [Line Items]
Gas and oil area acquired | a	71
Additional gas and oil area acquired | a	117
Clayton Williams Energy | Permian
Business Acquisition [Line Items]
Gas and oil area acquired | a	100
Clayton Williams Energy | Texas
Business Acquisition [Line Items]
Gas and oil area acquired | a				64