Acquisitions and Dispositions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		1 Months Ended	6 Months Ended	1 Months Ended		1 Months Ended		3 Months Ended	6 Months Ended	1 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013 Marcellus Acreage Natural_Gas_Producer acre	Jun. 30, 2014 Marcellus Acreage	Aug. 31, 2013 Elwood	Mar. 31, 2013 Elwood	Mar. 31, 2014 Electric Retail Energy Marketing Business	Jun. 30, 2013 Illinois Gas Contracts	Jun. 30, 2013 Brayton Point and Kincaid	Jun. 30, 2013 Brayton Point and Kincaid	Mar. 31, 2014 Acquisition of Solar Development Projects Project MW	May 31, 2014 Strata Solar Development LLC Project MW	May 31, 2014 EDF Renewable Development Inc MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of equity interests acquired											100.00%	100.00%	100.00%
Number of solar development projects											6	2
Acquisition price of solar development projects											$ 50	$ 2	$ 70
Expected cost of projects											450	70	75
Expected generation capacity of facilities											139	32	20
Proceeds from the sale of electric retail energy marketing business	187	0					187
Gain on sale							100	29
After tax gain on sale							57	18
Write off of goodwill							31	3
Sales price subject to post-closing adjustments								32
Proceeds from sale of Brayton Point, Kincaid and equity method investment in Elwood					465
Impairment of long-lived assets to be disposed of									11	48
Impairment charge, net of tax									6	28
Ownership%						50.00%
Number of natural gas producers			2
Number of acres of development rights			100,000
Payments provided by agreement subject to customary adjustments			200
Period for payments to DTI			9 years	9 years
Cash proceeds from sale of assets				16
Deferred revenue				$ 91