Pension, Postretirement, Other Postemployment, and Employee Savings Plan Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension, Postretirement, Other Postemployment, and Employee Savings Plan Benefits
Pension benefits	$ 327.8	$ 218.0
Postretirement benefits	347.8	334.9
Postemployment benefits	53.1	55.0
Total defined benefit obligations	728.7	607.9
Less: Amount recorded in other current liabilities	(30.7)	(33.2)
Total noncurrent defined benefit obligations	698.0	574.7
Age limit on January 1, 2002 under which future retirees' contributions are capped at the 2002 level (in years)	55
Reduction of Accumulated Postretirement Benefit Obligation due to the Medicare Prescription Drug Improvement and Modernization Act of 2003	30.3
Reduction of postretirement medical payments due to the Medicare Prescription Drug Improvement and Modernization Act of 2003	3.1
Net pension and postretirement benefit costs
Impact of funded status adjustments on Accumulated Other Comprehensive Income (Loss), Pre-tax	(175.2)	(84.6)
Impact of funded status adjustments on Accumulated Other Comprehensive Income (Loss), After-tax	(106.4)	(54.6)
Pension Plan
Net pension and postretirement benefit costs
Accumulated benefit obligation	1,672.0	1,493.0
Fair value of assets	1,487.7	1,408.1
Net (asset) unfunded obligation	184.3	84.9
Projected benefit obligation of pension liability increase (decrease) related to funded status adjustments	167.2	73.7
Change in benefit obligation
Benefit obligation, beginning balance	1,626.1	1,469.8
Service cost	50.3	37.9	50.8
Interest cost	87.4	84.7	101.1
Actuarial loss	146.2	124.0
Acquisition of Boston Generating plan	3.2
Plan amendments	(0.5)
Separation of CENG plans		(3.0)
Settlements	(6.0)	(5.2)
Special termination benefits		0.6
Benefits paid	(91.2)	(82.7)
Benefit obligation, ending balance	1,815.5	1,626.1	1,469.8
Qualified Pension Plans
Net pension and postretirement benefit costs
Accumulated benefit obligation		1,405.2
Fair value of assets		1,408.1
Net (asset) unfunded obligation		(2.9)
Qualified Pension Plans | Constellation Energy.
Net pension and postretirement benefit costs
Accumulated benefit obligation	1,567.0
Fair value of assets	1,484.3
Net (asset) unfunded obligation	82.7
Qualified Pension Plans | Boston Generating
Net pension and postretirement benefit costs
Accumulated benefit obligation	3.2
Fair value of assets	3.4
Net (asset) unfunded obligation	(0.2)
Non-qualified Pension Plans
Net pension and postretirement benefit costs
Accumulated benefit obligation	101.8	87.8
Net (asset) unfunded obligation	101.8	87.8
Post-retirement Plan
Net pension and postretirement benefit costs
Accumulated benefit obligation of postretirement liability increase (decrease) related to funded status adjustments	8.0	10.9
Change in benefit obligation
Benefit obligation, beginning balance	334.9	322.3
Service cost	2.9	2.4	6.3
Interest cost	17.1	17.7	22.6
Plan amendments	1.3	(3.3)
Plan participants' contributions	9.7	10.5
Actuarial loss (gain)	6.7	14.2	2.2
Special termination benefits	0.2	0.1
Benefits paid	(25.0)	(29.0)
Benefit obligation, ending balance	$ 347.8	$ 334.9	$ 322.3