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Information by Operating Segment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Generation	Jun. 30, 2010 Generation	Jun. 30, 2011 Generation	Jun. 30, 2010 Generation	Jun. 30, 2011 Generation CENG	Jun. 30, 2011 Generation CENG	Jun. 30, 2011 NewEnergy	Jun. 30, 2010 NewEnergy	Jun. 30, 2011 NewEnergy	Jun. 30, 2010 NewEnergy	Jun. 30, 2011 Regulated Electric	Jun. 30, 2010 Regulated Electric	Jun. 30, 2011 Regulated Electric	Jun. 30, 2010 Regulated Electric	Jun. 30, 2011 Regulated Gas	Jun. 30, 2010 Regulated Gas	Jun. 30, 2011 Regulated Gas	Jun. 30, 2010 Regulated Gas	Jun. 30, 2011 Holding Company and Other	Jun. 30, 2010 Holding Company and Other	Jun. 30, 2011 Holding Company and Other	Jun. 30, 2010 Holding Company and Other	Jun. 30, 2011 Eliminations	Jun. 30, 2010 Eliminations	Jun. 30, 2011 Eliminations	Jun. 30, 2010 Eliminations	Jun. 30, 2011 Boston Generating MW	Jun. 30, 2011 Boston Generating MW	Jan. 31, 2011 Boston Generating MW
Segment Reporting Information
Unaffiliated revenues	$ 3,359.8	$ 3,309.9	$ 6,930.0	$ 6,896.5	$ 256.4	$ 283.0	$ 515.9	$ 574.2			$ 2,447.8	$ 2,276.1	$ 4,802.7	$ 4,503.1	$ 546.9	$ 651.1	$ 1,197.0	$ 1,402.4	$ 108.2	$ 99.6	$ 414.4	$ 416.7	$ 0.5	$ 0.1		$ 0.1
Intersegment revenues					420.1	267.3	828.2	556.0			112.0	114.7	169.2	238.6	0.2		0.3		0.9	0.8	2.0	1.7					(533.2)	(382.8)	(999.7)	(796.3)
Total revenues	3,359.8	3,309.9	6,930.0	6,896.5	676.5	550.3	1,344.1	1,130.2			2,559.8	2,390.8	4,971.9	4,741.7	547.1	651.1	1,197.3	1,402.4	109.1	100.4	416.4	418.4	0.5	0.1		0.1	(533.2)	(382.8)	(999.7)	(796.3)
Net income	108.1	83.8	187.5	275.1	40.9	15.3	53.7	42.4			51.7	50.8	40.3	154.9	22.9	20.9	62.6	48.1	(6.3)	(3.9)	35.1	33.3	(1.1)	0.7	(4.2)	(3.6)
Net income (loss) attributable to common stock	99.2	72.6	169.6	264.1	40.9	15.3	53.7	42.4			46.1	42.9	29.0	150.5	20.5	18.4	57.7	43.0	(7.2)	(4.7)	33.4	31.8	(1.1)	0.7	(4.2)	(3.6)
Amortization of basis difference in CENG, after-tax									24.0	41.6
Impact of power purchase agreement, after-tax									30.3	57.3
Percentage of ownership interest in CENG (as a percent)									50.01%	50.01%
Gain on settlement with DOE					21.3		21.3
Underlying power generation capacity of acquired entities (in megawatts)																															2,950	2,950	2,950
Costs incurred related to pending merger with Exelon					9.5		9.5				4.3		4.3		4.1		4.1		1.4		1.4
Amortization of credit facility amendment fees in connection with the EDF transaction											1.5		3.0
Transaction fees incurred related to acquisition, after tax																															$ 0.1	$ 10.1