Commitments and Contingencies, Long-Term Power Sales Contracts, Litigation (Details 2) (USD $) In Millions, unless otherwise specified	1 Months Ended	2 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	9 Months Ended		12 Months Ended		1 Months Ended		9 Months Ended
	Aug. 31, 2011 Litigation, merger with Exelon Lawsuit	May 31, 2011 Litigation, merger with Exelon company Lawsuit	Aug. 31, 2011 Litigation, Securities Class Action complaint Lawsuit	Nov. 30, 2008 Litigation, Securities Class Action Lawsuit	Dec. 31, 2008 Litigation, Securities Class Action Lawsuit	Sep. 30, 2011 Litigation, Mercury Lawsuit Defendant Claimant	Sep. 30, 2011 Litigation, Asbestos Claimant	Dec. 31, 1999 Environmental Matters, Solid and Hazardous Waste Defendant	Sep. 30, 2011 Environmental Matters, Solid and Hazardous Waste	Jan. 31, 2009 Environmental Matters, Air Quality	Oct. 31, 2007 Environmental Matters, Water Quality	Sep. 30, 2011 Environmental Matters, Water Quality
Loss Contingencies
Number of merged companies		2
Number of lawsuits filed		12				70
Number of lawsuits for which motions are filed for consolidation		3
Number of lawsuits filed for breach of fiduciary duties	2
Number of lawsuits filed in District Court				3
Number of lawsuits transferred to other District Court for coordination			2		2
Number of amended complaints filed			3
Number of defendants in addition to BGE and Constellation Energy						11		19
Number of claimants						132	484
Compensatory damages sought per claimant						$ 20
Approximate ownership percentage in Keystone plant (as a percent)										20.99%
Minimum estimated clean-up costs for all potentially responsible parties									50
Maximum estimated clean-up costs for all potentially responsible parties									64
Consent decree penalty											1.0
Accrual for Environmental (Loss) Contingencies
Environmental loss contingencies											10.6	5.1
Environmental loss contingencies payments												5.5
Additional costs reasonably possible												$ 10