Acquisitions and Dispositions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended		1 Months Ended		6 Months Ended	1 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013 Marcellus Acreage Natural_Gas_Producer acre	Mar. 31, 2014 Marcellus Acreage	Aug. 31, 2013 Elwood	Mar. 31, 2014 Elwood	Mar. 31, 2014 Electric Retail Energy Marketing Business	Jun. 30, 2013 Electric Retail Energy Marketing Business	Jun. 30, 2013 Brayton Point and Kincaid	Mar. 31, 2014 Acquisition of Solar Development Projects Project MW
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										6
Acquisition price of solar development projects										$ 50
Expected cost of projects										450
Expected generation capacity of facilities										139
Proceeds from the sale of electric retail energy marketing business	187	0					187
Gain on sale								100
After tax gain on sale								57
Write off of goodwill								31
Proceeds from sale of Brayton Point, Kincaid and equity method investment in Elwood					465
Impairment of long-lived assets to be disposed of									37
Impairment charge, net of tax									22
Equity method investment, ownership percentage						50.00%
Number of natural gas producers			2
Number of acres of development rights			100,000
Cash proceeds from sale of assets				12
Deferred revenue				$ 89