Other Events (Details 5) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended		1 Months Ended		12 Months Ended				1 Months Ended	3 Months Ended			12 Months Ended				3 Months Ended		1 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended		1 Months Ended
	Nov. 30, 2009	Dec. 31, 2011	Dec. 31, 2010 Y	Dec. 31, 2009	Nov. 30, 2010 Comprehensive agreement with EDF	Mar. 31, 2009 14% Senior Note held by MidAmerican	Feb. 28, 2010 7.00% Fixed-Rate Notes, due April 1, 2012	Nov. 30, 2009 7.00% Fixed-Rate Notes, due April 1, 2012	Dec. 31, 2010 7.00% Fixed-Rate Notes, due April 1, 2012	Dec. 31, 2009 7.00% Fixed-Rate Notes, due April 1, 2012	Dec. 31, 2009 District chilled water facility	Dec. 31, 2009 Other Marketable Securities	Mar. 31, 2009 CEP	Jun. 30, 2009 Shipping JV	Dec. 31, 2011 CENG	Sep. 30, 2010 CENG	Dec. 31, 2011 CENG	Dec. 31, 2010 CENG	Dec. 31, 2009 CENG Y	Nov. 06, 2009 CENG	Dec. 31, 2010 Coal-fired generating plants Entity	Sep. 30, 2010 Coal-fired generating plants Entity	Jan. 31, 2010 Baltimore Gas and Electric Company	Dec. 31, 2011 Baltimore Gas and Electric Company	Dec. 31, 2009 Baltimore Gas and Electric Company	Aug. 31, 2010 Mammoth Lakes Geothermal Generating Facility	Sep. 30, 2010 Mammoth Lakes Geothermal Generating Facility	Sep. 30, 2010 UNE	Nov. 30, 2010 Constellation Energy; Comprehensive agreement with EDF	Jan. 31, 2011 Constellation Energy; 7.00% Fixed-Rate Notes, due April 1, 2012	Dec. 31, 2010 Constellation Energy; 7.00% Fixed-Rate Notes, due April 1, 2012
Asset Impairment
Equity method investments with fair value decline substantially below book value																					1	3
Impairment charge		$ 891,000,000	$ 2,476,800,000	$ 124,700,000							$ 20,000,000	$ 500,000	$ 14,000,000	$ 59,000,000	$ 824,200,000	$ 2,300,000,000	$ 824,200,000				$ 8,400,000	$ 50,000,000			$ 20,000,000			$ 143,400,000
Merger costs, pre-tax		117,900,000																						30,300,000
Percentage interest sold (as a percent)														50.00%												50.00%
Equity method investment closing price				5,100,000,000									10,000,000		2,200,000,000	2,900,000,000	2,200,000,000
Carrying value, equity method investment		2,272,800,000	3,174,200,000										24,000,000		3,000,000,000	5,200,000,000	3,000,000,000											143,400,000
Impairment charge net of noncontrolling interest											12,000,000
Impairment charge noncontrolling interest impact											8,000,000
Impairment losses on nuclear decommissioning trust assets				62,600,000
Loss on redemption of Zero Coupon Senior Notes, pre-tax	(16,000,000)			(16,000,000)				(16,000,000)	(51,600,000)	(16,000,000)																				(5,000,000)
Retirement of aggregate principal amount of fixed term notes							486,500,000
Early retirement of debt, premium, percentage (as a percent)							11.00%
Term of power purchase agreement with CENG (in years)			5																5
Fair value of power purchase agreement with CENG				800,000,000															800,000,000	800,000,000
Amortization period of power purchase agreements (in years)																			2
Net economic value of power purchase agreement with CENG																			400,000,000
Pre-tax net economic value realized of power purchase agreement with CENG																	200,400,000	185,600,000
Net cash proceeds of sale of interest in equity method investment					140,000,000																		20,900,000			72,500,000
Constellation Energy common stock received on sale of interest in equity method investment																													75,200,000
Gain on sale of interest in equity method investments		93,800,000			202,000,000																						38,000,000
Other costs				31,200,000
Loss on Redemption of Zero Coupon Senior Notes
Loss on redemption of Zero Coupon Senior Notes, pre-tax	(16,000,000)			(16,000,000)				(16,000,000)	(51,600,000)	(16,000,000)																				(5,000,000)
Merger, Termination and Strategic Alternatives Costs
Merger termination and strategic alternatives costs				145,800,000
Interest rate (as a percent)				3.50%		14.00%																									7.00%
Workforce Reduction Costs,
Workforce reduction costs				$ 12,600,000