Pension, Postretirement, Other Postemployment, and Employee Savings Plan Benefits (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of net periodic pension and postretirement benefit cost
Termination benefits excluded from net periodic defined benefit costs		$ 0.1
Term to which expected asset gains and losses are ratably applied when determining net periodic pension benefit cost (in years)	5
Pension Plan
Change in plan assets
Fair value of plan assets at beginning of period	1,408.1	1,058.1
Actual return on plan assets	89.5	148.8
Employer contribution	84.3	289.1
Acquisition of Boston Generating Plan	3
Settlements	(6.0)	(5.2)
Benefits paid	(91.2)	(82.7)
Fair value of plan assets at end of period	1,487.7	1,408.1	1,058.1
Components of net periodic pension and postretirement benefit cost
Service cost	50.3	37.9	50.8
Interest cost	87.4	84.7	101.1
Expected return on plan assets	(114.9)	(101.8)	(118.9)
Amortization of unrecognized prior service cost	3.9	3.9	10.9
Recognized net actuarial loss (gain)	44.6	34.4	38.3
Amount capitalized as construction cost	(10.6)	(10.2)	(10.2)
Net periodic pension and postretirement benefit cost	60.7	48.9	72.0
Settlement charges excluded from net periodic pension benefit costs	4.0	1.5	9.0
Termination benefits excluded from net periodic defined benefit costs	0.2	0.6	0.1
Pension Plan | Baltimore Gas and Electric Company
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	34.0	30.9	27.9
Qualified Pension Plans
Change in plan assets
Employer contribution	75.6
Fair value of plan assets at end of period	1,487.7
Qualified Pension Plans | Baltimore Gas and Electric Company
Change in plan assets
Employer contribution	53.6
Post-retirement Plan
Change in plan assets
Employer contribution	15.3	18.5
Plan participants' contributions	9.7	10.5
Benefits paid	(25.0)	(29.0)
Components of net periodic pension and postretirement benefit cost
Service cost	2.9	2.4	6.3
Interest cost	17.1	17.7	22.6
Amortization of transition obligation	1.7	2.1	2.1
Amortization of unrecognized prior service cost	(2.7)	(2.6)	(3.4)
Recognized net actuarial loss (gain)	1.5	0.4	2.2
Amount capitalized as construction cost	(5.3)	(5.4)	(6.3)
Net periodic pension and postretirement benefit cost	15.2	14.6	23.5
Post-retirement Plan | Baltimore Gas and Electric Company
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	$ 17.5	$ 17.2	$ 18.7