Dispositions (Narrative) (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended		3 Months Ended
	Jun. 30, 2013 Elwood	Mar. 31, 2013 Elwood	Jun. 30, 2013 Illinois Gas Contracts	Mar. 31, 2013 Brayton Point, Kincaid, and Elwood Scenario, Forecast Subsequent Event	Jun. 30, 2013 Brayton Point and Kincaid	Mar. 31, 2013 Brayton Point and Kincaid
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales price			$ 32	$ 472
Gain on sale			29
After tax gain on sale			18
Write off of goodwill			3
Equity method investment, ownership percentage	50.00%	50.00%
Impairment of long-lived assets to be disposed of					11	37
Impairment charge, net of tax					$ 6	$ 22