Other Events (Details) (USD $)	1 Months Ended		3 Months Ended			12 Months Ended
	Apr. 30, 2011	Nov. 30, 2009	Dec. 31, 2011	Mar. 31, 2010	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2010
Other Events
Gain on sale of 49.99% membership interest in our nuclear generation and operation business (CENG) to EDF, pre-tax								$ 7,445,600,000
Gain on sale of 49.99% membership interest in our nuclear generation and operation business (CENG) to EDF, after-tax								4,456,100,000
Basis difference in investment in CENG			453,100,000		3,900,000,000	453,100,000		3,900,000,000	1,500,000,000
Impact of power purchase agreement with CENG, pre-tax						(200,400,000)	(185,600,000)
Impact of power purchase agreement with CENG, after-tax						(118,500,000)	(113,300,000)
Amortization of basis difference in CENG, pre-tax						(153,100,000)	(195,200,000)	(29,600,000)
Amortization of basis difference in CENG, after-tax						(90,500,000)	(117,500,000)	(17,800,000)
Gain on settlements with U. S. Department of Energy, pre-tax						93,800,000
Gain on settlements with U. S. Department of Energy, after-tax						57,300,000
Gain on divestitures, pre-tax						57,300,000	240,000,000
Gain on divestitures, after-tax						32,700,000	146,000,000
Merger costs, pre-tax						(117,900,000)
Merger costs, after-tax						(70,900,000)
Transaction fees for Boston Generating acquisition, pre-tax						(15,500,000)
Transaction fees for Boston Generating acquisition, after-tax						(9,900,000)
Net (loss) gain on divestitures, pre-tax						57,300,000	245,800,000	(468,800,000)
Net (loss) gain on divestitures, after-tax								(293,200,000)
Impairment losses and other costs, pre-tax						(891,000,000)	(2,476,800,000)	(124,700,000)
Impairment losses and other costs, after-tax						(530,200,000)	(1,487,100,000)	(96,200,000)
International coal contract dispute settlement, pre-tax							56,600,000
International coal contract dispute settlement, after-tax							35,400,000
Deferred income tax expense relating to federal subsidies for providing post-employment prescription drug benefits				(8,800,000)			(8,800,000)
Loss on early retirement of 2012 Notes, pre-tax							(51,600,000)
Loss on early retirement of 2012 Notes, after-tax				(30,900,000)			(30,900,000)
Fair value of power purchase agreement with CENG					800,000,000			800,000,000
Impairment of nuclear decommissioning trust assets, pre-tax								(62,600,000)
Impairment of nuclear decommissioning trust assets, after-tax								(46,800,000)
Loss on redemption of Zero Coupon Senior Notes, pre-tax		(16,000,000)						(16,000,000)
Loss on redemption of Zero Coupon Senior Notes, after-tax								(10,000,000)
Maryland PSC order-BGE residential customer credits, pre-tax					(112,400,000)			(112,400,000)
Maryland PSC order-BGE residential customer credits, after-tax								(67,100,000)
Merger termination and strategic alternatives costs, pre-tax								(145,800,000)
Merger termination and strategic alternatives costs, after-tax								(13,800,000)
Workforce reduction costs, pre-tax								(12,600,000)
Workforce reduction costs, after-tax								(9,300,000)
Total other items, pre-tax						(1,226,800,000)	(2,612,600,000)	6,473,100,000
Total other items, after-tax						$ (730,000,000)	$ (1,576,200,000)	$ 3,901,900,000
Percentage of decline in natural gas prices			19.00%
Number of common shares of Constellation Energy to be cancelled and converted into the right to receive for each share of Exelon (in shares)	0.93