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Retirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2011
Retirement Benefits Tables [Abstract]  
Schedule Of Pension And Other Postretirement Participation [Text Block]
  Operating Company
Name of Plan:Generation ComEd PECO BSC
Qualified Pension Plans:       
Exelon Corporation Retirement ProgramX X X X
Exelon Corporation Cash Balance Pension PlanX X X X
Exelon Corporation Pension Plan for        
 Bargaining Unit EmployeesX X   X
Exelon New England Union Employees Pension PlanX      
Exelon Employee Pension Plan for Clinton,       
 TMI and Oyster CreekX   X X
         
Non-Qualified Pension Plans:       
Exelon Corporation Supplemental Pension Benefit Plan        
 and 2000 Excess Benefit PlanX X X X
Exelon Corporation Supplemental Management        
 Retirement PlanX X X X
         
Other Postretirement Benefit Plans:       
PECO Energy Company Retiree Medical PlanX   X X
Exelon Corporation Health Care ProgramX X   X
AmerGen Energy Company Postretirement Welfare PlanX   X X
Defined Benefit Plan Change In Benefit Obligation RollForward [Text Block]
       Other
 Pension Benefits Postretirement Benefits
 2011 2010 2011 2010
Change in benefit obligation:           
Net benefit obligation at beginning of year$12,524 $11,482 $3,874 $3,658
Service cost 212  190  142  124
Interest cost 649  660  207  214
Plan participants’ contributions 0  0  25  16
Actuarial loss 807  831  4  49
Special termination benefits 0  0   —   1
Gross benefits paid (654)  (639)   (201)   (198)
Federal subsidy on benefits paid  —   —   11   10
            
Net benefit obligation at end of year$13,538 $12,524 $4,062 $3,874
            
Change in plan assets:           
Fair value of net plan assets at beginning of year$8,859 $7,839 $1,655 $1,476
Actual return on plan assets 1,003  893  29  158
Employer contributions 2,094  766  277  203
Plan participants’ contributions 0  0  25  16
Benefits paid (a) (654)  (639)  (189)  (198)
            
Fair value of net plan assets at end of year$11,302 $8,859 $1,797 $1,655
Defined Benefit Plan Amounts Recognized In Balance Sheet [Text Block]
        Other
  Pension Benefits Postretirement Benefits
  2011 2010 2011 2010
Other current liabilities$42 $7 $2 $1
Pension obligations 2,194  3,658   —   —
Non-pension postretirement benefit obligations  —   —  2,263  2,218
             
Unfunded status (net benefit obligation less            
 net plan assets)$2,236 $3,665 $2,265 $2,219
Defined Benefit Plan Pension Plans With Projected Benefit Obligations And Accumulated Benefit Obligations In Excess Of Plan Assets [Text Block]
 PBO and ABO in excess of plan assets
 2011 2010
      
Projected benefit obligation$13,538 $12,524
Accumulated benefit obligation 12,616  11,697
Fair value of net plan assets 11,302  8,859
Schedule of Defined Benefit Plans Disclosures Text Block
   Pension Benefits Other Postretirement Benefits
   2011 2010 2009 2011 2010 2009
Components of net periodic benefit                  
 cost:                  
Service cost$212 $190 $178 $142 $124 $113
Interest cost 649  660  651  207  214  205
Expected return on assets (939)  (799)  (778)  (111)  (109)  (94)
Amortization of:                 
 Transition obligation  —   —   —  9  9  9
 Prior service cost (credit) 14  14  14  (38)  (56)  (56)
 Actuarial loss 331  254  197  66  74  87
Curtailment/settlement charges  —  5  6   —   —   —
Special termination benefits  —   —   —   —   1   4
                    
Net periodic benefit cost$267 $324 $268 $275 $257 $268
Prescription Drug Benefit Reduction In Accumulated Postretirement Benefit Obligation For Subsidy [Text Block]
 2011 2010 2009
         
Amortization of the actuarial experience loss$3 $9 $11
Reduction in current period service cost 9  10  9
Reduction in interest cost on the APBO 16  19  18
         
Total effect of subsidy on net periodic postretirement benefit cost$28 $38 $38
Changes In Plan Assets And Benefit Obligations Recognized In OCI And Regulatory Assets [Text Block]
   Pension Benefits Other Postretirement Benefits
   2011 2010 2009 2011 2010 2009
Changes in plan assets and benefit                 
 obligations recognized in AOCI                  
 and regulatory assets:                 
Current year actuarial (gain) loss$ 744 $ 737 $ (94) $ 74 $ — $ (154)
Amortization of actuarial loss  (331)   (254)   (197)   (66)   (74)   (87)
Current year prior service cost  —   —   2   —   —   —
Amortization of prior service cost                 
 (credit)  (14)   (14)   (14)   38   56   56
Amortization of transition obligation  —   —   —   (9)   (9)   (9)
Settlements  —   (5)   (6)   —   —   —
                    
Total recognized in AOCI and                  
 regulatory assets (a)$ 399 $ 464 $ (309) $ 37 $ (27) $ (194)

Of the $399 million related to pension benefits, $181 million and $218 million were recognized in AOCI and regulatory assets, respectively, during 2011. Of the $37 million related to other postretirement benefits, $13 million and $24 million were recognized in AOCI and regulatory assets, respectively, during 2011. Of the $464 million related to pension benefits, $310 million and $154 million were recognized in AOCI and regulatory assets, respectively, during 2010. Of the $(27) million related to other postretirement benefits, $(9) million and $(18) million were recognized in AOCI and regulatory assets, respectively, during 2010. Of the $(309) related to pension benefits, $(204) million and $(105) million were recognized in AOCI and regulatory assets, respectively, during 2009. Of the $(194) million related to other postretirement benefits, $(85) million and $(109) million were recognized in AOCI and regulatory assets, respectively, during 2009.

 
Changes In Plan Assets And Benefit Obligations Not Recognized In OCI And Regulatory Assets [Text Block]
 Pension Benefits Other Postretirement Benefits
 2011 2010 2011 2010
Transition obligation$ — $ — $ 11 $ 20
Prior service cost (credit) 90  104  (16)  (54)
Actuarial loss 6,729  6,316  963  955
            
Total (a)$6,819 $6,420 $958 $921

 

(a)       Of the $6,819 million related to pension benefits, $4,311 million and $2,508 million are included in AOCI and regulatory assets, respectively, at December 31, 2011. Of the $958 million related to other postretirement benefits, $475 million and $483 million are included in AOCI and regulatory assets, respectively, at December 31, 2011. Of the $6,420 million related to pension benefits, $4,129 million and $2,291 million are included in AOCI and regulatory assets, respectively, at December 31, 2010. Of the $921 million related to other postretirement benefits, $462 million and $459 million are included in AOCI and regulatory assets, respectively, at December 31, 2010.

 
Defined Benefit Plan Amounts That Will Be Amortized From Accumulated Other Comprehensive Income Loss And Regulatory Assets In Next Fiscal Year [Text Block]
  Pension Benefits Other Postretirement Benefits
Transition obligation $ — $10
Prior service cost (credit)   14  (12)
Actuarial loss   407  69
       
Total (a) $421 $67

 

(a)       Of the $421 million related to pension benefits at December 31, 2011, $252 million and $169 million are expected to be amortized from AOCI and regulatory assets in 2012, respectively. Of the $67 million related to other postretirement benefits at December 31, 2011, $32 million and $35 million are expected to be amortized from AOCI and regulatory assets in 2012, respectively.

 
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation [Text Block]
   Pension Benefits Other Postretirement Benefits
   2011 2010 2009 2011 2010 2009
Discount rate4.74% 5.26% 5.83% 4.80% 5.30% 5.83%
Rate of compensation increase3.75% 3.75% 4.00% 3.75% 3.75% 4.00%
Mortality tableIRS required mortality table for 2012 funding valuation IRS required mortality table for 2011 funding valuation IRS required mortality table for 2010 funding valuation IRS required mortality table for 2012 funding valuation IRS required mortality table for 2011 funding valuation IRS required mortality table for 2010 funding valuation
Health care cost trend on covered chargesN/A N/A N/A 6.50% decreasing to ultimate trend of 5.00% in 2017 7.00% decreasing to ultimate trend of 5.00% in 2015 7.50% decreasing to ultimate trend of 5.00% in 2015
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost [Text Block]
   Pension Benefits Other Postretirement Benefits
   2011 2010 2009 2011 2010 2009
Discount rate5.26% 5.83% 6.09% 5.30% 5.83% 6.09%
Expected return on plan assets8.00% (a) 8.50% (a) 8.50% (a) 7.08% (a) 7.83% (a) 8.10% (a)
Rate of compensation increase3.75% 4.00% 4.00% 3.75% 4.00% 4.00%
Mortality tableIRS required mortality table for 2011 funding valuation IRS required mortality table for 2010 funding valuation IRS required mortality table for 2009 funding valuation IRS required mortality table for 2011 funding valuation IRS required mortality table for 2010 funding valuation IRS required mortality table for 2009 funding valuation
Health care cost trend on covered chargesN/A N/A N/A 7.00% decreasing to ultimate trend of 5.00% in 2015 7.50% decreasing to ultimate trend of 5.00% in 2015 7.50% decreasing to ultimate trend of 5.00% in 2014

 

(a)       Not applicable to pension and other postretirement benefit plans that do not have any plan assets.

 
Defined Benefit Plan Effect Of One Percentage Point Change On Accumulated Postretirement Benefit Obligation [Text Block]
Effect of a one percentage point increase in assumed health care cost trend:  
 on 2011 total service and interest cost components$75
 on postretirement benefit obligation at December 31, 2011 686
Effect of a one percentage point decrease in assumed health care cost trend:  
 on 2011 total service and interest cost components (57)
 on postretirement benefit obligation at December 31, 2011 (521)
Pension And Other Postretirement Benefit Contributions [Text Block]
 Pension Benefits Other Postretirement Benefits
 2011 2010 2009 2011 (a) 2010 (a) 2009 (a)
Generation$954 $356 $201 $121 $94 $69
ComEd 873  260  164  108  60  53
PECO 110  73  31  28  35  22
BSC 157  77  45  20  14  13
                  
Exelon$2,094 $766 $441 $277 $203 $157

 

(a)       The Registrants present the cash contributions above net of Federal subsidy payments received on each of their respective Consolidated Statements of Cash Flows. Exelon, Generation, ComEd and PECO received Federal subsidy payments of $11 million, $5 million, $4 million and $1 million, respectively, in 2011, $10 million, $5 million, $3 million and $2 million, respectively, in 2010, and $10 million, $5 million, $3 million and $1 million, respectively, in 2009.

 
Defined Benefit Plan Estimated Future Benefit Payments [Text Block]
    Other Postretirement
 Pension Benefits Benefits (a)
2012$786 $186
2013 680  178
2014 692  186
2015 723  194
2016 750  204
2017 through 2021 4,335  1,206
      
Total estimated future benefit payments through 2021$7,966 $2,154
Schedule Of Pension And Other Postretirement Benefit Costs [Text Block]
               
For the Year Ended December 31, Generation  ComEd  PECO  BSC (a)  Exelon
                
2011 $249 $213 $32 $48 $542
2010  268  215  46  52  581
2009  240  192  47  57  536

___________________       

 

(a)       These amounts primarily represent amounts billed to Exelon's subsidiaries through intercompany allocations.

 
Defined Benefit Plan Weighted Average Asset Allocations And Target Allocations [Text Block]
Pension Plans   Percentage of Plan Assets 
    at December 31, 
Asset CategoryTarget Allocation 2011 2010
          
Equity securities30 - 40%  32%  45%
Fixed income securities35 - 55%  47   41 
Alternative investments (a)20 - 30%  21   14 
          
Total    100%  100%
          
Other Postretirement Benefit Plans   Percentage of Plan Assets 
    at December 31, 
Asset CategoryTarget Allocation 2011 2010
          
Equity securities40 - 50%  37%  54%
Fixed income securities35 - 45%  53   45 
Alternative investments (a)10 - 20%  10   1 
          
Total    100%  100%

 

(a) Alternative investments include private equity, hedge funds and real estate.

 
Defined Benefit Plan Fair Value Of Plan Assets [Text Block]
At December 31, 2011 (a) (b)Level 1 Level 2 Level 3 Total
             
Pension plan assets           
 Cash equivalents$ 8 $0 $0 $ 8
 Equity securities:           
  Individually held   1,985  0  0   1,985
  Commingled funds  0   858  0   858
  Mutual funds 0   389  0   389
 Equity securities subtotal  1,985   1,247  0   3,232
 Fixed income securities:           
  Debt securities issued by the U.S. Treasury and           
  other U.S. government corporations and agencies   1,616   48  0   1,664
  Debt securities issued by states of the United States            
  and by political subdivisions of the states  0   88  0   88
  Foreign debt securities 0   224  0   224
  Corporate debt securities  0   2,561  0   2,561
  Federal agency mortgage-backed securities  0   156  0   156
  Non-Federal agency mortgage-backed securities  0   28  0   28
  Commingled funds  0   202  0   202
  Mutual funds 0   277  0   277
 Fixed income securities subtotal  1,616   3,584  0   5,200
 Private equity 0  0   672   672
 Hedge funds 0  0   1,525   1,525
 Real estate  207  125   229   561
Pension plan assets subtotal  3,816   4,956   2,426   11,198
             
Other postretirement benefit plan assets           
 Cash equivalents  73  0  0   73
 Equity securities:           
  Individually held   110  0  0   110
  Commingled funds  0   415  0   415
  Mutual funds 0   171  0   171
 Equity securities subtotal  110   586  0   696
 Fixed income securities:           
  Debt securities issued by the U.S. Treasury and           
  other U.S. government corporations and agencies   26   3  0   29
  Debt securities issued by states of the United States            
  and by political subdivisions of the states  0   93  0   93
  Foreign debt securities 0   4  0   4
  Corporate debt securities  0   41  0   41
  Federal agency mortgage-backed securities  0   34  0   34
  Non-Federal agency mortgage-backed securities  0   7  0   7
  Commingled funds  0   385  0   385
  Mutual funds 0   256  0   256
 Fixed income securities subtotal  26   823  0   849
 Private equity 0  0   1   1
 Hedge funds 0  0   157   157
 Real estate  4  1   7   12
Other postretirement benefit plan assets subtotal  213   1,410   165   1,788
             
Total pension and other postretirement            
 benefit plan assets$ 4,029 $ 6,366 $ 2,591 $ 12,986
             
At December 31, 2010 (a) (b)Level 1 Level 2 Level 3 Total
             
Pension plan assets           
 Cash equivalents$ 2 $0 $0 $ 2
 Equity securities:           
  Individually held   1,528  0  0   1,528
  Commingled funds  0   2,161  0   2,161
  Mutual funds 0   326  0   326
 Equity securities subtotal  1,528   2,487  0   4,015
 Fixed income securities:           
  Debt securities issued by the U.S. Treasury and           
  other U.S. government corporations and agencies   1,144   20  0   1,164
  Debt securities issued by states of the United States            
  and by political subdivisions of the states  0   15  0   15
  Foreign debt securities 0   73  0   73
  Corporate debt securities  0   312  0   312
  Federal agency mortgage-backed securities  0   226  0   226
  Non-Federal agency mortgage-backed securities  0   82  0   82
  Commingled funds  0   1,036  0   1,036
  Mutual funds 0   666  0   666
 Fixed income securities subtotal  1,144   2,430  0   3,574
 Private equity 0  0   536   536
 Hedge funds 0  0   329   329
 Real estate  178  0   179   357
Pension plan assets subtotal  2,852   4,917   1,044   8,813
             
Other postretirement benefit plan assets           
 Cash equivalents 0  0  0  0
 Equity securities:           
  Individually held   225  0  0   225
  Commingled funds  0   447  0   447
  Mutual funds 0   218  0   218
 Equity securities subtotal  225   665  0   890
 Fixed income securities:           
  Debt securities issued by the U.S. Treasury and           
  other U.S. government corporations and agencies   25   1  0   26
  Debt securities issued by states of the United States            
  and by political subdivisions of the states  0   100  0   100
  Foreign debt securities 0   1  0   1
  Corporate debt securities  0   13  0   13
  Federal agency mortgage-backed securities  0   41  0   41
  Non-Federal agency mortgage-backed securities  0   7  0   7
  Commingled funds 0   225  0   225
  Mutual funds 0   331  0   331
 Fixed income securities subtotal  25   719  0   744
 Hedge funds 0  0   5   5
 Real estate  8  0   8   16
Other postretirement benefit plan assets subtotal  258   1,384   13   1,655
             
Total pension and other postretirement            
 benefit plan assets$ 3,110 $ 6,301 $ 1,057 $ 10,468

 

(a)       See Note 8 - Fair Value of Assets and Liabilities for a description of levels within the fair value hierarchy.

(b)       The total fair value of pension and other postretirement benefit plan assets excludes $55 million and $21 million of interest and dividends receivable and $57 million and $25 million related to pending sales transactions at December 31, 2011 and 2010, respectively. Additionally, the table excludes collateral fund assets of $17 million and $47 million and collateral liabilities of $17 million and $47 million at December 31, 2011 and 2010, respectively, in connection with the benefit plans' participation in securities lending programs.

 
Defined Benefit Plan Fair Value Of Plan Assets Unobservable Input Reconciliation [Text Block]
           
 Hedge funds Private equity Real estate Total
              
Pension Assets           
Balance as of January 1, 2011$ 329 $ 536 $ 179 $ 1,044
Actual return on plan assets:            
 Relating to assets still held at the reporting date  (26)   50   46   70
Purchases, sales and settlements:           
 Purchases  1,222   121   13   1,356
 Sales 0  (1)  (9)  (10)
 Settlements 0  (34)  0  (34)
              
Balance as of December 31, 2011$ 1,525 $ 672 $ 229 $ 2,426
              
Other Postretirement Benefits           
Balance as of January 1, 2011$ 5 $0 $ 8 $ 13
Actual return on plan assets:            
 Relating to assets still held at the reporting date  (3)  0   (1)  (4)
Purchases, sales and settlements:           
 Purchases  155   1  0   156
 Sales 0  0  0  0
 Settlements 0  0  0  0
              
Balance as of December 31, 2011$ 157 $ 1 $ 7 $ 165
              
           
 Hedge funds Private equity Real estate Total
              
Pension Assets           
Balance as of January 1, 2010$0 $ 450 $ 156 $ 606
Actual return on plan assets:            
 Relating to assets still held at the reporting date  14   37   13   64
Purchases, sales and settlements:           
 Purchases  315   67   14   396
 Sales 0  0  (4)  (4)
 Settlements 0  (18)  0  (18)
Transfers into (out of) Level 3 0  0  0  0
              
Balance as of December 31, 2010$ 329 $ 536 $ 179 $ 1,044
              
Other Postretirement Benefits           
Balance as of January 1, 2010$0 $0 $0 $0
Actual return on plan assets:            
 Relating to assets still held at the reporting date 0  0   2   2
Purchases, sales and settlements:           
 Purchases  5  0   2   7
 Sales 0  0  0  0
 Settlements 0  0  0  0
Transfers into (out of) Level 3 (a) 0  0   4   4
              
Balance as of December 31, 2010$ 5 $0 $ 8 $ 13

(a)       Commingled fund investments determined to be illiquid during the year were transferred into Level 3.

 
Schedule Of Defined Contributions [Text Block]
For the Year Ended December 31,  Exelon  Generation  ComEd  PECO
             
2011 $78 $40 $22 $9
2010  81  42  22  9
2009  70  36  20  8