Acquisitions and Divestitures (Summary of Consideration Paid and Fair Value of Assets Acquired and Liabilities Assumed) (Details) - WPX Property Acquisition [Member] - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2017
Business Acquisition [Line Items]
Cash	$ 270	$ 270
Proved natural gas and oil properties		31
Unproved natural gas and oil properties		114
Intangible asset		109
Gathering system		22
Other		1
Total assets acquired		277
Asset retirement obligations		(7)
Total liabilities assumed		(7)
Total assets acquired and liabilities assumed		$ 270