Quarterly Financial Data (Unaudited) (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended								9 Months Ended	12 Months Ended			3 Months Ended							3 Months Ended		12 Months Ended		3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Exelon Corporation	Sep. 30, 2011 Exelon Corporation	Jun. 30, 2011 Exelon Corporation	Dec. 31, 2011 Boston Generating MW	Jun. 30, 2011 Boston Generating MW	Mar. 31, 2011 Boston Generating MW	Jan. 03, 2011 Boston Generating MW	Dec. 31, 2011 CEP	Sep. 30, 2011 CEP	Dec. 31, 2010 NewEnergy	Dec. 31, 2011 NewEnergy Exelon Corporation	Sep. 30, 2010 Mammoth Lakes Geothermal Generating Facility	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
Quarterly Financial Data:
After-tax charge for amortization of basis difference in investment in CENG										$ 90.5	$ 117.5	$ 17.8													$ 22.6	$ 26.3	$ 24.0	$ 17.6	$ 23.3	$ 31.5	$ 37.0	$ 25.7
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
After-tax loss for the early retirement of 2012 Notes								(30.9)			(30.9)
Transaction fees incurred related to acquisition, after tax										15.5							0.1	10.0
Tax benefit related to acquisition																0.2
Underlying power generation capacity of acquired entities (in megawatts)																2,950	2,950	2,950	2,950
After-tax amortization of credit facility amendment fees in connection with the EDF transaction	1.3	1.5	1.5	1.5	4.9	2.9	2.9	2.9
After-tax gain on sale of additional interests in CEP and certain working interests in upstream natural gas properties																				18.4
After-tax charges related to our pending merger with Exelon													46.5	5.1	19.3								16.1
Gain on settlement with DOE, after-tax	36.0		21.3						35.5	93.8
After-tax charge for incremental operating expenses incurred related to Hurricane Irene		24.6
Gain on sales of CEP equity, after-tax																					14.3
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											0.1
Loss on redemption of debt, after-tax												(10.0)
Gain on sale of interest in equity method investment, after-tax																								24.7
After-tax gain on the settlement of international coal contract dispute					35.4
After-tax gain on comprehensive agreement with EDF					121.3
After-tax charge for impairment associated with certain equity method investments	530.2					1,465.3
After-tax charge for impairment and an adjustment to income tax expenses associated with certain equity method investments					21.8
After-tax impairment charge (benefit) related to nuclear decommissioning trust fund investments												$ 46.8