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Pension And Other Postretirement Benefits (Narrative) (Detail) (USD $)
 12 Months Ended 0 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
Employees
Dec. 31, 2011
Plans
Employees
Dec. 31, 2010
Dec. 31, 2012
Potomac Electric Power Co [Member]
Dec. 31, 2011
Potomac Electric Power Co [Member]
Dec. 31, 2010
Potomac Electric Power Co [Member]
Dec. 31, 2012
Delmarva Power & Light Co/De [Member]
Dec. 31, 2011
Delmarva Power & Light Co/De [Member]
Dec. 31, 2010
Delmarva Power & Light Co/De [Member]
Dec. 31, 2012
Atlantic City Electric Co [Member]
Dec. 31, 2011
Atlantic City Electric Co [Member]
Dec. 31, 2010
Atlantic City Electric Co [Member]
Dec. 31, 2012
Subsidiaries [Member]
Dec. 31, 2011
Subsidiaries [Member]
Dec. 31, 2012
Other Postretirement Benefits [Member]
Dec. 31, 2011
Other Postretirement Benefits [Member]
Dec. 31, 2010
Other Postretirement Benefits [Member]
Jan. 09, 2013
Other Postretirement Benefits [Member]
Subsequent Event [Member]
Dec. 31, 2012
Other Postretirement Benefits [Member]
Potomac Electric Power Co [Member]
Dec. 31, 2011
Other Postretirement Benefits [Member]
Potomac Electric Power Co [Member]
Dec. 31, 2012
Other Postretirement Benefits [Member]
Delmarva Power & Light Co/De [Member]
Dec. 31, 2011
Other Postretirement Benefits [Member]
Delmarva Power & Light Co/De [Member]
Jan. 09, 2013
Other Postretirement Benefits [Member]
Delmarva Power & Light Co/De [Member]
Subsequent Event [Member]
Dec. 31, 2012
Other Postretirement Benefits [Member]
Atlantic City Electric Co [Member]
Dec. 31, 2011
Other Postretirement Benefits [Member]
Atlantic City Electric Co [Member]
Jan. 09, 2013
Other Postretirement Benefits [Member]
Atlantic City Electric Co [Member]
Subsequent Event [Member]
Dec. 31, 2011
Retiree [Member]
Employees
Dec. 31, 2011
Union Employees [Member]
Employees
Dec. 31, 2012
Retirement Plan [Member]
Dec. 31, 2011
Retirement Plan [Member]
Dec. 31, 2010
Retirement Plan [Member]
Dec. 31, 2012
Pension Benefits [Member]
Dec. 31, 2011
Pension Benefits [Member]
Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]                                                                    
Medicare Part D subsidy receipts $ 4,000,000 $ 2,000,000                                                                
Retirement Plan assets 2,000,000,000 1,700,000,000                                                                
Accumulated benefit obligation 2,300,000,000 2,000,000,000                         775,000,000 750,000,000 704,000,000                             2,494,000,000 2,124,000,000 1,970,000,000
Net loss to be amortized from AOCL or regulatory assets into net periodic benefit cost                             15,000,000                                 68,000,000    
Prior service cost to be amortized from AOCL or regulatory assets into net periodic benefit cost                             4,000,000                                 1,000,000    
Expected long-term rate of return on plan assets 7.25% 7.75%                         7.25% 7.75% 8.00%                             7.25% 7.75% 8.00%
Medical trend rate assumed for next fiscal year 8.00% 8.00%                         8.00% 8.00%                                     
Direction and pattern of medical trend rate declining 0.5% per year to a rate of 5.0% for 2018 to 2019 and beyond declining 0.5% per year to a rate of 5.0% for 2017 to 2018 and beyond                                                                
Number of employees affected by changes to retirement plan 750 1,400                                                 400 1,000            
Number of pre-existing supplemental retirement plans   2                                                                
Plan Asset Allocation, Equity                             62.00% 62.00%                         30.00% 36.00% 53.00%      
Plan Asset Allocation, Fixed Income                             36.00% 36.00%                         62.00% 56.00% 40.00%      
Plan Asset Allocation, Other                                                         8.00% 8.00% 7.00%      
Unfunded capital commitments 15,000,000 28,000,000                                                                
Pension contributions 200,000,000 110,000,000 100,000,000 85,000,000 40,000,000 0 85,000,000 40,000,000 0 30,000,000 30,000,000 0           20,000,000 85,000,000 40,000,000 85,000,000 40,000,000 10,000,000 30,000,000 30,000,000 30,000,000                
OPEB contributions       5,000,000 7,000,000 10,000,000 7,000,000 6,000,000 9,000,000 7,000,000 7,000,000 8,000,000 13,000,000 13,000,000                                        
Percentage of participants vested 100.00%                                                                  
Defined contribution plan matching contributions 12,000,000 11,000,000 11,000,000                                                              
Pension and other postretirement net periodic benefit cost       39,000,000 43,000,000 40,000,000 23,000,000 23,000,000 28,000,000 24,000,000 21,000,000 23,000,000                                            
Prepaid pension expense       353,000,000 289,000,000   232,000,000 162,000,000   88,000,000 71,000,000                                              
Other postretirement benefit obligations $ 454,000,000 $ 469,000,000   $ 66,000,000 $ 66,000,000   $ 22,000,000 $ 22,000,000   $ 34,000,000 $ 31,000,000       $ 454,000,000 $ 469,000,000