Investments (Details 4) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended		1 Months Ended		3 Months Ended		12 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended			12 Months Ended
	Nov. 30, 2009	Dec. 31, 2009	Nov. 30, 2010	Nov. 30, 2010 EDF	Oct. 31, 2010 EDF	Dec. 31, 2010 EDF	Oct. 26, 2010 EDF	Dec. 31, 2011 CENG	Nov. 06, 2009 CENG	Dec. 31, 2010 CENG Power purchase agreement (PPA)	Sep. 30, 2011 CEP	Dec. 31, 2010 Qualifying facilities and domestic project	Nov. 03, 2010 UNE EDF	Nov. 03, 2010 UNE Constellation Energy.	Dec. 31, 2010 EDF in PPA EDF Power purchase agreement (PPA)
Equity Method Investment Financial Statement Reported Amounts
Percentage of joint venture interest in UNE sold (as a percent)							50.00%
Cash received from sale of joint venture interest	$ 3,500.0	$ 3,528.7			$ 140.0
Number of own shares received for sale of joint venture interest				0.1	2.4
Fair value of own shares received for sale of joint venture interest			72.4	2.8
Gain (loss) on sale of joint venture interest						$ 202.0					$ 14.3
Number of own shares to be received upon CENG's transfer to UNE of potential new nuclear sites						1.0
Percentage of ownership interest in equity investment		50.01%						50.01%	50.01%				49.99%	50.01%
Purchase of nuclear output of CENG (as a percent)								50.01%		50.01%
Purchase of nuclear output by EDF (as a percent)															49.99%
Maximum percentage of voting interest held by Generation business in 15 operating domestic energy projects (as a percent)												50.00%
Number of operating domestic energy projects held with up to a 50% voting interest												15
Number of operating domestic energy projects that are "qualifying facilities"												13