Property, Plant and Equipment (Asset Retirement Obligations) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 247,000,000	$ 209,000,000
Additional liabilities incurred	12,000,000	43,000,000
Asset Retirement Obligation, Other [Line Items]	1,000,000	(5,000,000)
Liabilities settled	(4,000,000)	(6,000,000)
Accretion expense	15,000,000	13,000,000
Change in estimated cash flows	86,000,000	(7,000,000)
Ending balance	357,000,000	247,000,000
Asset retirement obligation, legally restricted	$ 1,000,000	$ 2,000,000