Information by Operating Segment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 Exelon Corporation Generation	Sep. 30, 2011 Exelon Corporation Generation	Sep. 30, 2011 CENG Generation	Sep. 30, 2011 CENG Generation	Sep. 30, 2011 Generation	Sep. 30, 2010 Generation	Sep. 30, 2011 Generation	Sep. 30, 2010 Generation	Sep. 30, 2011 Exelon Corporation NewEnergy	Sep. 30, 2011 Exelon Corporation NewEnergy	Sep. 30, 2011 NewEnergy	Sep. 30, 2010 NewEnergy	Sep. 30, 2011 NewEnergy	Sep. 30, 2010 NewEnergy	Sep. 30, 2011 Exelon Corporation Regulated Electric	Sep. 30, 2011 Exelon Corporation Regulated Electric	Sep. 30, 2011 Regulated Electric	Sep. 30, 2010 Regulated Electric	Sep. 30, 2011 Regulated Electric	Sep. 30, 2010 Regulated Electric	Sep. 30, 2011 Exelon Corporation Regulated Gas	Sep. 30, 2011 Exelon Corporation Regulated Gas	Sep. 30, 2011 Regulated Gas	Sep. 30, 2010 Regulated Gas	Sep. 30, 2011 Regulated Gas	Sep. 30, 2010 Regulated Gas	Sep. 30, 2011 Holding Company and Other	Sep. 30, 2010 Holding Company and Other	Sep. 30, 2011 Holding Company and Other	Sep. 30, 2010 Holding Company and Other	Sep. 30, 2011 Eliminations	Sep. 30, 2010 Eliminations	Sep. 30, 2011 Eliminations	Sep. 30, 2010 Eliminations	Sep. 30, 2011 CENG	Sep. 30, 2011 Boston Generating mW	Jan. 03, 2011 Boston Generating mW
Segment Reporting Information
Unaffiliated revenues	$ 3,521.1	$ 3,968.9	$ 10,451.1	$ 10,865.4					$ 311.4	$ 328.7	$ 827.3	$ 902.9			$ 2,488.9	$ 2,786.1	$ 7,291.6	$ 7,289.2			$ 638.7	$ 776.3	$ 1,835.7	$ 2,178.7			$ 81.9	$ 77.7	$ 496.3	$ 494.4	$ 0.2	$ 0.1	$ 0.2	$ 0.2
Intersegment revenues									416.8	258.9	1,245.0	814.9			181.4	134.2	350.6	372.8			0.2	0.1	0.5	0.1			2.1	2.0	4.1	3.7					(600.5)	(395.2)	(1,600.2)	(1,191.5)
Total revenues	3,521.1	3,968.9	10,451.1	10,865.4					728.2	587.6	2,072.3	1,717.8			2,670.3	2,920.3	7,642.2	7,662.0			638.9	776.4	1,836.2	2,178.8			84.0	79.7	500.4	498.1	0.2	0.1	0.2	0.2	(600.5)	(395.2)	(1,600.2)	(1,191.5)
Net income (loss)	97.9	(1,375.0)	285.4	(1,099.9)					30.1	(1,420.1)	83.8	(1,377.7)			64.4	13.4	104.7	168.3			11.7	37.6	74.3	85.7			(10.1)	(5.8)	25.0	27.5	1.8	(0.1)	(2.4)	(3.7)
Net income (loss) attributable to common stock	73.7	(1,406.5)	243.3	(1,142.4)					30.1	(1,420.1)	83.8	(1,377.7)			43.5	(14.8)	72.5	135.7			9.2	35.0	66.9	78.0			(10.9)	(6.5)	22.5	25.3	1.8	(0.1)	(2.4)	(3.7)
Amortization of basis difference in CENG, after-tax							26.3	68.0
Impact of power purchase agreement, after-tax							31.3	88.6
Percentage of ownership interest in CENG (as a percent)							50.01%	50.01%																															50.01%
Gain on settlement with DOE, after-tax									0		21.3
Transaction fees incurred related to acquisition, after tax									0		10.1
Underlying power generation capacity of acquired entities (in megawatts)																																								2,950	2,950
Costs incurred related to pending merger with Exelon, after-tax					2.5	12.0							1.3	5.6					1.0	5.1					0.3	1.7
Amortization of credit facility amendment fees in connection with the EDF transaction, after-tax															1.5		4.4
Total operating expenses related to Hurricane Irene, after-tax																							$ 35.9