Information by Operating Segment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended						9 Months Ended	12 Months Ended				3 Months Ended							3 Months Ended									3 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Nov. 06, 2011	Dec. 31, 2011 Exelon Corporation	Sep. 30, 2011 Exelon Corporation	Jun. 30, 2011 Exelon Corporation	Jun. 30, 2011 Boston Generating MW	Mar. 31, 2011 Boston Generating MW	Dec. 31, 2011 Boston Generating MW	Jan. 03, 2011 Boston Generating MW	Dec. 31, 2011 CENG	Sep. 30, 2011 CENG	Jun. 30, 2011 CENG	Mar. 31, 2011 CENG	Dec. 31, 2010 CENG	Sep. 30, 2010 CENG	Jun. 30, 2010 CENG	Mar. 31, 2010 CENG	Nov. 06, 2009 CENG	Sep. 30, 2010 Mammoth Lakes Geothermal Generating Facility	Nov. 30, 2009 Generation	Dec. 31, 2011 Generation	Dec. 31, 2010 Generation	Dec. 31, 2009 Generation	Dec. 31, 2011 Generation Exelon Corporation	Dec. 31, 2011 Generation CENG Unit	Dec. 31, 2010 Generation CENG	Dec. 31, 2009 Generation CENG	Dec. 31, 2010 Generation Mammoth Lakes Geothermal Generating Facility	Dec. 31, 2011 NewEnergy	Dec. 31, 2010 NewEnergy	Dec. 31, 2009 NewEnergy	Dec. 31, 2011 NewEnergy Exelon Corporation	Dec. 31, 2011 Regulated Electric	Dec. 31, 2010 Regulated Electric	Dec. 31, 2009 Regulated Electric	Dec. 31, 2011 Regulated Electric Exelon Corporation	Dec. 31, 2011 Regulated Gas	Dec. 31, 2010 Regulated Gas	Dec. 31, 2009 Regulated Gas	Dec. 31, 2011 Regulated Gas Exelon Corporation	Dec. 31, 2011 Holding Company and Other	Dec. 31, 2010 Holding Company and Other	Dec. 31, 2009 Holding Company and Other	Dec. 31, 2011 Eliminations	Dec. 31, 2010 Eliminations	Dec. 31, 2009 Eliminations
Segment Reporting Information
Unaffiliated revenues								$ 13,758.2	$ 14,340.0	$ 15,598.8																				$ 1,122.3	$ 1,189.2	$ 664.2						$ 9,649.0	$ 9,692.6	$ 11,345.8		$ 2,320.7	$ 2,752.1	$ 2,820.7		$ 664.5	$ 704.9	$ 753.8		$ 1.7	$ 1.2	$ 14.3
Intersegment revenues																														1,595.4	1,055.1	2,110.0						471.2	428.8	163.4		0.7	0.2			7.2	4.5	4.5				0.1	(2,074.5)	(1,488.6)	(2,278.0)
Total revenues								13,758.2	14,340.0	15,598.8																				2,717.7	2,244.3	2,774.2						10,120.2	10,121.4	11,509.2		2,321.4	2,752.3	2,820.7		671.7	709.4	758.3		1.7	1.2	14.4	(2,074.5)	(1,488.6)	(2,278.0)
Depreciation, depletion, accretion, and amortization								589.3	519.5	651.4																				187.4	137.7	238.9						89.4	83.7	82.7		226.5	205.2	218.1		45.6	44.0	44.0		40.4	48.9	67.7
Fixed charges								265.4	277.8	350.1																				128.8	142.0	166.5						9.4	3.0	39.7		103.4	106.3	113.3		23.2	24.0	26.0		0.1	(0.2)	2.4	0.5	2.7	2.2
Income Tax (Benefit) Expense								(230.9)	(665.7)	2,986.8																				(300.7)	(873.1)	3,107.1						(8.8)	106.5	(179.1)		50.1	72.6	50.9		23.4	24.5	17.1		5.1	3.8	(9.2)
Net (loss) income								(306.8)	(931.8)	4,503.4																				(441.1)	(1,255.3)	4,766.7						2.8	176.2	(348.2)		93.6	110.0	79.1		42.1	37.6	25.5		(4.2)	(0.3)	(19.7)
Net income (loss) attributable to common stock								(340.3)	(982.6)	4,443.4																				(441.1)	(1,255.3)	4,766.7						(17.5)	138.6	(402.3)		83.8	99.8	68.9		38.7	34.6	22.5		(4.2)	(0.3)	(12.4)
Segment assets	19,412.6			20,018.5				19,412.6	20,018.5	23,544.4																				8,738.6	9,789.6	12,402.1						4,136.1	3,836.2	4,167.5		5,506.7	5,287.4	4,994.6		1,480.3	1,379.9	1,413.4		854.2	858.0	4,573.7	(1,303.3)	(1,132.6)	(4,006.9)
Capital expenditures								1,160.5	1,056.7	1,595.0																				159.7	327.4	1,039.2						322.9	127.2	116.8		535.4	499.1	373.0		142.5	103.0	66.0
Amortization of basis difference in CENG, after-tax								(90.5)	(117.5)	(17.8)									(22.6)	(26.3)	(24.0)	(17.6)	(23.3)	(31.5)	(37.0)	(25.7)								(90.5)	(117.5)	(17.8)
Impact of power purchase agreement, after-tax								118.5	113.3										29.9	31.3	30.3	27.0	29.6	28.9	29.1	25.7								118.5	113.3
Gain on settlement with DOE, after-tax	36.0		21.3				35.5	93.8																						57.3
Transaction fees incurred related to acquisition, after tax								15.5							0.1	10.0														9.9
Underlying power generation capacity of acquired entities (in megawatts)															2,950	2,950	2,950	2,950
Costs incurred related to pending merger with Exelon, after-tax												46.5	5.1	19.3																			37.0								16.1				13.3				4.5
Gain on divestitures, after-tax								32.7	146.0																													32.7			32.7
Amortization of credit facility amendment fees in connection with the EDF transaction, after-tax																																						5.8
Total incremental operating expenses related to Hurricane Irene, after-tax		24.6																																								24.6
Impairment charges on equity method investment	891.0			8.4	2,468.4																										1,487.1
Gain on sale of interest in equity method investment, after-tax																												24.7									24.7
Gain on comprehensive agreement with EDF				121.3																											121.3
International coal contract dispute settlement, after-tax									35.4																														35.4
After-tax charge for the deferred income tax expense impact relating to federal subsidies for providing post-employment prescription drug benefits						8.8			8.8																						0.8								0.1				3.1								4.8
Percent of membership interest in CENG sold										49.99%	49.99%																49.99%									49.99%
After-tax gain on sale of interest in CENG to EDF										4,456.1																										4,456.1
After-tax loss on redemption of debt										10.0																					30.9	10.0
Merger termination and strategic alternatives costs, after-tax										13.8																						9.7								4.1
Impairment of nuclear decommissioning trust assets, after-tax										46.8																			46.8
Reclassification of losses on previously designated cash-flow hedges from AOCI into earnings and earnings that are no longer part of our core business, after-tax																																								371.9
Impairment charges and other costs, after-tax																														530.2										84.7												11.5
Workforce reduction costs, after-tax										9.3																														9.3
Accrual of a residential customer credit, after-tax																																												$ 56.7				$ 10.4
Number of nuclear generating units owned, operated and maintained by CENG																																		5