Merger, Acquisitions and Divestitures - Sales Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]
Sales Proceeds	$ 1,241	$ 151	$ 321
Net Book Value of Assets Sold	(993)	(156)	(297)
Asset Retirement Obligations Associated with Assets Sold	7	8	48
Goodwill Allocated to Assets Sold	0	(4)	(7)
Other Closing Adjustments	(17)	1	8
Gain on Divestitures	$ 238	$ 0	$ 73