Clayton Williams Energy Acquisition - Purchase Price Allocation (Details) - USD ($) $ in Millions		12 Months Ended
	Apr. 24, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Plus: Cash Consideration Paid to Clayton Williams Energy Stockholders		$ 616	$ 0	$ 0
Clayton Williams Energy
Business Acquisition [Line Items]
Fair value of common stock issued	$ 1,876
Plus: Cash Consideration Paid to Clayton Williams Energy Stockholders	637
Total purchase price	2,513
Accounts Payable	99
Other Current Liabilities	38
Long-Term Deferred Tax Liability	509
Long-Term Debt	595
Asset Retirement Obligation	63
Total purchase price plus liabilities assumed	$ 3,817