Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities on Consolidated Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations	$ 396		$ 369
Assets of discontinued operations
Accounts receivable and unbilled revenue	0		5
Materials and supplies	0		5
Property and plant, net	0		142
Accumulated deferred income taxes, net	15	[1]	13	[1]
Total assets of discontinued operations	15		165		1,611
Liabilities of discontinued operations
Accounts payable and other current obligations	1		5
Asset retirement obligations	32	[2]	40	[2]
Total liabilities of discontinued operations	33		45
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations	$ 32		$ 31

[1] (a)The December 31, 2014 balance primarily consists of deferred income tax assets related to the abandoned Meredosia and Hutsonville energy centers.

[2] (b)Includes AROs associated with the abandoned Meredosia and Hutsonville energy centers of $32 million and $31 million at December 31, 2014 and 2013, respectively.