Acquisitions and Dispositions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended		6 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Jun. 30, 2013	Jun. 30, 2013	Aug. 31, 2013	Nov. 30, 2014 MW	Dec. 31, 2014 MW	Sep. 30, 2014 MW Project	Jan. 31, 2015 mi
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of electric retail energy marketing business	$ 187	$ 0	$ 0
Asset disposition adjustment	42	44
Impairment charge			1,600
Electric Retail Energy Marketing Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of electric retail energy marketing business				187
Gain on sale				100
After-tax gain on sale				57
Asset disposition adjustment				31
Illinois Gas Contracts
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale					29
After-tax gain on sale					18
Asset disposition adjustment					3
Sales price					32	32
Brayton Point and Kincaid
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment charge			1,600			48
Long-lived assets impairment charges, net of tax						28
Loss from discontinued operation		17
Loss on discontinued operation, net of tax		18
Loss from Discontinued Operations | Brayton Point and Kincaid
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset disposition adjustment		16
Elwood
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership percentage							50.00%
Proceeds from sale of equity method investment							465
Elwood | Other Income
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of investment							35
After tax gain on sale of investment							25
Recurrent Energy Development Holdings, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of equity interests acquired				100.00%
Number of solar development projects acquired				6
Purchase price				50
Estimated project cost once constructed				446
Generation capacity of solar development projects				139
CSI Project Holdco, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of equity interests acquired								100.00%
Purchase price								79
Estimated project cost once constructed								80
Generation capacity of solar development projects								20
EDF Renewable Development, Inc.
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 acquired										2
Purchase price									71	175
Estimated project cost once constructed	73								73	185
Generation capacity of solar development projects									20	42
CGT | Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of equity interests acquired											100.00%
Purchase price											$ 495
Length of FERC regulated interstate natural gas pipeline											1,500