Pension, Postretirement, Other Postemployment, and Employee Savings Plan Benefits (Details 7) (USD $) In Millions, unless otherwise specified			12 Months Ended							12 Months Ended															1 Months Ended
	Nov. 06, 2011	Dec. 31, 2009	Dec. 31, 2011 Other Postemployment Benefits	Dec. 31, 2010 Other Postemployment Benefits	Dec. 31, 2009 Other Postemployment Benefits	Dec. 31, 2011 Employee Savings Plan	Dec. 31, 2010 Employee Savings Plan	Dec. 31, 2009 Employee Savings Plan	Nov. 06, 2009 Employee Savings Plan	Dec. 31, 2011 Baltimore Gas and Electric Company Other Postemployment Benefits	Dec. 31, 2010 Baltimore Gas and Electric Company Other Postemployment Benefits	Dec. 31, 2009 Baltimore Gas and Electric Company Other Postemployment Benefits	Dec. 31, 2011 Baltimore Gas and Electric Company Employee Savings Plan	Dec. 31, 2010 Baltimore Gas and Electric Company Employee Savings Plan	Dec. 31, 2009 Baltimore Gas and Electric Company Employee Savings Plan	Dec. 31, 2011 Nonregulated businesses Employee Savings Plan	Dec. 31, 2010 Nonregulated businesses Employee Savings Plan	Dec. 31, 2009 Nonregulated businesses Employee Savings Plan	Dec. 31, 2011 Pension Plan	Dec. 31, 2010 Pension Plan	Dec. 31, 2011 Qualified Pension Plans	Dec. 31, 2011 Qualified Pension Plans Baltimore Gas and Electric Company	Dec. 31, 2011 Post-retirement Plan	Dec. 31, 2010 Post-retirement Plan	Nov. 30, 2009 Level 3 Employee Savings Plan plan
Contributions and Benefit Payments
Qualified pension plan contributions																			$ 84.3	$ 289.1	$ 75.6	$ 53.6	$ 15.3	$ 18.5
Accelerated contribution for calendar year 2012																					75.0
Other Postemployment Benefits:
Other postemployment benefits expense			3.0	9.9	5.3					2.8	7.6	4.4
Other postemployment benefits discount rate (as a percent)			3.00%	4.00%
Employee Savings Plan Benefits
Number of sponsored defined contribution plans until November 6, 2009																									2
Percent of membership interest in CENG sold	49.99%	49.99%							49.99%
Matching contributions made towards employee savings plan benefits						$ 16.7	$ 16.2	$ 20.5					$ 6.6	$ 6.3	$ 5.7	$ 10.1	$ 9.9	$ 14.8