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Income Taxes - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Mar. 23, 2016
Jun. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jul. 01, 2017
Jun. 29, 2017
Dec. 15, 2016
Jun. 30, 2016
Mar. 24, 2016
Dec. 15, 2015
Dec. 31, 2014
Dec. 31, 1999
Income Taxes [Line Items]                                          
Document Period End Date                     Dec. 31, 2017                    
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                   21.00% 35.00% 35.00% 35.00%                
LimitationonDeductibleInterestExpense                   100.00%                      
AllowableExpensingForCostOfQualifiedProperty                   30.00%                      
BonusDepreciationPercentage                   50.00%                      
Unrecognized Tax Benefits $ 743     $ 916         $ 916   $ 743 $ 916 $ 1,078             $ 1,829  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     (2) 106 0                
Unrecognized Tax Benefits, Interest on Income Taxes Expense         $ 94                                
Unrecognized Tax Benefits, Period Increase (Decrease)                     (196) [1] (21) (116)                
Taxes accrued 373     723         723   373 723                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued (17)     (106)         (106)   (17) (106)                  
Tax positions for which there is uncertainty about the timing of tax benefits 7     83         83   7 83 415                
Unrecognized Tax Benefits that Would Impact Effective Tax Rate 523     633         633   523 $ 633 $ 538                
Income Tax Expense (Benefit), Intraperiod Tax Allocation     $ 28                                    
Like kind exchange [Member]                                          
Income Taxes [Line Items]                                          
Income Taxes Receivable             $ 369                            
Payments for Other Taxes 20                   950                    
Deferred gain on sale of property                                         $ 1,200
Unrecognized Tax Benefits, Income Tax Penalties Expense                     90                    
Decrease in Unrecognized Tax Benefits is Reasonably Possible   $ 39           $ 39                          
Expected non-cash charge to earnings             26               $ 50            
Unrecognized Tax Benefits, Period Increase (Decrease)             2                            
Investment Agreement, Early Termination Fee       1,250                                  
Taxes accrued   335 106         335                 $ 360        
Unrecognized Tax Benefits, Income Tax Penalties Accrued 1,300                   $ 1,300                    
Deferred               350                          
Tax Adjustments, Settlements, and Unusual Provisions               150                          
Exelon Generation Co L L C [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% 35.00%                
Unrecognized Tax Benefits 468     490         490   $ 468 $ 490 $ 534             1,357  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     (17) [1] (20) (112)                
Taxes accrued 444     239         239   444 239                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   0 0                  
Tax positions for which there is uncertainty about the timing of tax benefits                         20                
Unrecognized Tax Benefits that Would Impact Effective Tax Rate 461     483         483   461 483 509                
Allocation of federal tax benefit under tax sharing agreement 102     94         94   $ 102 $ 94 $ 57                
Commonwealth Edison Co [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% 35.00%                
Unrecognized Tax Benefits 2     (12)         (12)   $ 2 $ (12) $ 142             149  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 86 0                
Unrecognized Tax Benefits, Interest on Income Taxes Expense         $ 64                                
Unrecognized Tax Benefits, Period Increase (Decrease)                     0 [1] 0 0                
Taxes accrued 60     330         330   60 330                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     (86)         (86)   0 $ (86)                  
Tax positions for which there is uncertainty about the timing of tax benefits                         $ 142                
Unrecognized Tax Benefits that Would Impact Effective Tax Rate 2                   2                    
Commonwealth Edison Co [Member] | Like kind exchange [Member]                                          
Income Taxes [Line Items]                                          
Decrease in Unrecognized Tax Benefits is Reasonably Possible   2           2                          
Expected non-cash charge to earnings             23               $ 15            
Unrecognized Tax Benefits, Period Increase (Decrease)             $ 8                            
Investment Agreement, Early Termination Fee                     300                    
Taxes accrued     86                                    
Deferred                     $ 300                    
PECO Energy Co [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% 35.00%                
Unrecognized Tax Benefits 0     0         0   $ 0 $ 0 $ 0             44  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     0 [1] 0 0                
Taxes accrued 15     4         4   15 4                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   0 0                  
Allocation of federal tax benefit under tax sharing agreement 16     18         18   $ 16 $ 18 $ 16                
Baltimore Gas and Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% 35.00%                
Unrecognized Tax Benefits 120     120         120   $ 120 $ 120 $ 120     $ 120     $ 120 0  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     0 [1] 0 0                
Taxes accrued 71     78         78   71 78                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   0 0                  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate                         120                
Allocation of federal tax benefit under tax sharing agreement 10     8         8   $ 10 $ 8 $ 7                
Potomac Electric Power Company [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% 35.00%                
Unrecognized Tax Benefits 59     80         80   $ 59 $ 80 $ 8     59     59 0  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     (21) [1] 0 0                
Taxes accrued 68     48         48   68 48                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   $ 0 0                  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate       21         21     $ 21 $ 8                
Delmarva Power and Light Company [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% [2] 35.00%                
Unrecognized Tax Benefits 21     37         37   $ 21 $ 37 $ 3     21     21 0  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     (16) [1] 0 0                
Taxes accrued 4     4         4   4 4                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   $ 0 0                  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate       16         16     $ 16 $ 3                
Atlantic City Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     35.00% 35.00% [2] 35.00%                
Unrecognized Tax Benefits 14     22         22   $ 14 $ 22 $ 0     14       0  
Unrecognized Tax Benefits, Income Tax Penalties Expense                     0 0 0                
Unrecognized Tax Benefits, Period Increase (Decrease)                     (22) [1] 0 0                
Taxes accrued 5     9         9   5 9                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   $ 0 0                  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate       22         $ 22     22                  
Pepco Holdings LLC [Member]                                          
Income Taxes [Line Items]                                          
Fresh-Start Adjustment, Increase (Decrease), Deferred Income Tax Liabilities, Noncurrent                                   $ 59      
Pepco Holdings LLC [Member] | Successor [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                 35.00% [2]   35.00%                    
Unrecognized Tax Benefits 125     172         $ 172   $ 125 172 $ 22     $ 94          
Unrecognized Tax Benefits, Period Increase (Decrease)                     (61) [1] 0                  
Taxes accrued 61     65         65   61 65                  
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0     0         0   0 0                  
Unrecognized Tax Benefits that Would Impact Effective Tax Rate 32                   32                    
Allocation of federal tax benefit under tax sharing agreement 7                   7                    
Pepco Holdings LLC [Member] | Predecessor [Member]                                          
Income Taxes [Line Items]                                          
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent           35.00%             35.00%                
Unrecognized Tax Benefits                         $ 22           $ 80 $ 702  
Unrecognized Tax Benefits, Period Increase (Decrease)                         0                
Unrecognized Tax Benefits that Would Impact Effective Tax Rate       $ 93         93     93 11                
Internal Revenue Service (IRS) [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     (469) 607 566                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 146                   146                    
Internal Revenue Service (IRS) [Member] | Exelon Generation Co L L C [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     (2,003) (247) 16                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 19                   19                    
Internal Revenue Service (IRS) [Member] | Commonwealth Edison Co [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     523 379 310                
Internal Revenue Service (IRS) [Member] | PECO Energy Co [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     28 72 69                
Internal Revenue Service (IRS) [Member] | Baltimore Gas and Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     101 88 126                
Internal Revenue Service (IRS) [Member] | Potomac Electric Power Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     114 136 126                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 21                   21                    
Internal Revenue Service (IRS) [Member] | Delmarva Power and Light Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     82 97 73                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 16                   16                    
Internal Revenue Service (IRS) [Member] | Atlantic City Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     34 22 27                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 22                   22                    
Internal Revenue Service (IRS) [Member] | Pepco Holdings LLC [Member] | Successor [Member]                                          
Income Taxes [Line Items]                                          
Deferred                 283   250                    
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 59                   59                    
Internal Revenue Service (IRS) [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]                                          
Income Taxes [Line Items]                                          
Deferred           $ 10             103                
State and Local Jurisdiction [Member]                                          
Income Taxes [Line Items]                                          
Deferred Tax Assets, State Taxes   272 8         272                          
Unrecognized Tax Benefits, Increase Resulting from Settlements with Taxing Authorities     20                                    
Deferred                     161 79 208                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 683                   683                    
Unrecognized Tax Benefits that Would Impact Effective Tax Rate 462                   462                    
Other, net   250                                      
Deferred State and Local Income Tax Expense (Benefit)               20                          
IllinoisCorporateIncomeTaxRate             7.75%             9.50%              
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     0 (1) 49                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 469                   469                    
Other, net   20                                      
Deferred State and Local Income Tax Expense (Benefit)               $ 20                          
State and Local Jurisdiction [Member] | Commonwealth Edison Co [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     136 63 45                
Other, net   $ 270                                      
State and Local Jurisdiction [Member] | PECO Energy Co [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     (9) 5 20                
State and Local Jurisdiction [Member] | Baltimore Gas and Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     49 31 39                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 120                   120                    
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     13 16 24                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 59                   59                    
Other, net     8                                    
State and Local Jurisdiction [Member] | Delmarva Power and Light Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     13 12 1                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 21                   21                    
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]                                          
Income Taxes [Line Items]                                          
Deferred                     4 $ 0 5                
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit 14                   14                    
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member] | Successor [Member]                                          
Income Taxes [Line Items]                                          
Deferred                 $ 13   32                    
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit $ 94                   $ 94                    
Other, net     $ 8                                    
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]                                          
Income Taxes [Line Items]                                          
Deferred           $ 7             $ 32                
[1] Predecessor Exelon Generation ComEd PECO BGE PHI Pepco DPL ACEUnrecognized tax benefits at January 1, 2015$1,829 $1,357 $149 $44 $— $702 $— $— $—Increases based on tax positions related to 2015108 — — — 106 — — — —Decreases based on tax positions related to 2015— — — — — — — — —Change to positions that only affect timing(705) (659) (7) (44) — (688) — — —Increases based on tax positions prior to 201579 65 — — 14 11 8 3 —Decreases based on tax positions prior to 2015(116) (112) — — — — — — —Decreases from settlements with taxing authorities(31) (31) — — — — — — —Decreases from expiration of statute of limitations(86) (86) — — — (3) — — —Unrecognized tax benefits at December 31, 2015$1,078 $534 $142 $— $120$22$8$3$—
[2] DPL and ACE recognized a loss before income taxes for the year ended December 31, 2016, and PHI recognized a loss before income taxes for the period of March 24, 2016, through December 31, 2016. As a result, positive percentages represent an income tax benefit for the periods presented.