Pension and Postretirement Benefits (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Baltimore Gas and Electric Company | Pension Plan
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	$ 10.5	$ 7.9	$ 28.6	$ 22.7
Pension Plan
Components of net periodic pension and postretirement benefit cost
Service cost	13.4	9.7	39.8	28.3
Interest cost	24.0	21.7	71.2	63.1
Expected return on plan assets	(31.7)	(26.0)	(94.0)	(75.9)
Recognized net actuarial loss (gain)	12.3	8.8	36.4	25.6
Amortization of prior service cost	1.1	1.0	3.2	2.9
Amount capitalized as construction cost	(2.7)	(2.5)	(8.8)	(7.3)
Net periodic pension and postretirement benefit cost	16.4	12.7	47.8	36.7
Settlement charges excluded from net periodic pension benefit costs			4.0	1.5
Non-qualified Pension Plans
Components of net periodic pension and postretirement benefit cost
Expected benefit payments	9.4		9.4
Baltimore Gas and Electric Company | Post-retirement Plan
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	4.7	3.8	14.2	13.2
Post-retirement Plan
Components of net periodic pension and postretirement benefit cost
Service cost	0.8	0.6	2.3	1.9
Interest cost	4.6	3.9	13.9	13.6
Amortization of transition obligation	0.4	0.5	1.4	1.6
Recognized net actuarial loss (gain)	0.4	0.1	1.2	0.3
Amortization of prior service cost	(0.7)	(0.6)	(2.2)	(2.0)
Amount capitalized as construction cost	(1.2)	(1.2)	(4.3)	(4.1)
Net periodic pension and postretirement benefit cost	4.3	3.3	12.3	11.3
Expected benefit payments to be made for retiree health and life insurance costs in 2011	$ 22.6		$ 22.6