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Income Taxes - Reconciliation to Effective Tax Rate (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 23, 2016
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     0.60% 2.30% 3.30% [1]
Qualified nuclear decommissioning trust fund income     (1.90%) 3.80% 3.40%
Amortization of investment tax credit, including deferred taxes on basis difference     (1.20%) (0.90%) (1.20%)
Plant basis differences     (3.50%) [2] (1.70%) (4.90%)
Production tax credits and other credits     (2.20%) (1.80%) (3.60%)
Noncontrolling interests     (1.00%)   (0.20%)
Like-Kind Exchange     (8.30%) (1.20%)  
Statute of limitations expiration         (0.40%)
Penalties         1.90%
Merger expenses       (3.60%) 5.60%
FitzPatrick bargain purchase gain       (2.20%)  
Tax Cuts and Jobs Act of 2017 [3]     0.90% (33.10%)  
Other     1.00% 0.10% (0.70%) [4]
Effective income tax rate     5.40% (3.30%) 38.20%
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 35  
Exelon Generation Co L L C [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     (16.60%) 2.90% 3.20% [1]
Qualified nuclear decommissioning trust fund income     (11.80%) 9.90% 7.90%
Amortization of investment tax credit, including deferred taxes on basis difference     (6.50%) (2.10%) (2.30%)
Plant basis differences     (0.00%) [2] (0.00%) [2] (0.00%)
Production tax credits and other credits     (13.50%) (4.70%) (8.30%)
Noncontrolling interests     (6.10%)   (0.60%)
Like-Kind Exchange     0.00% 0.00%  
Statute of limitations expiration         (1.70%)
Penalties         0.00%
Merger expenses       (1.20%) 1.10%
FitzPatrick bargain purchase gain       (5.60%)  
Tax Cuts and Jobs Act of 2017 [3]     2.70% (128.30%)  
Other     1.30% (0.50%) (1.40%) [4]
Effective income tax rate     (29.50%) (94.60%) 32.90%
Commonwealth Edison Co [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     8.30% 5.70% 5.60% [1]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00%
Amortization of investment tax credit, including deferred taxes on basis difference     (0.20%) (0.20%) (0.30%)
Plant basis differences     (0.20%) [2] 0.30% [2] (0.60%)
Production tax credits and other credits     (0.00%) (0.00%) (0.00%)
Noncontrolling interests     (0.00%)   (0.00%)
Like-Kind Exchange     (9.10%) 1.30%  
Statute of limitations expiration         0.00%
Penalties         4.50%
Merger expenses       0.00% 0.00%
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     (0.10%) 0.10%  
Other     0.50% 0.20% 0.10% [4]
Effective income tax rate     20.20% 42.40% 44.30%
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 3  
PECO Energy Co [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     (2.60%) 0.60% 1.30% [1]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00%
Amortization of investment tax credit, including deferred taxes on basis difference     (0.10%) (0.10%) (0.10%)
Plant basis differences     (14.10%) [2] (13.80%) [2] (9.60%)
Production tax credits and other credits     (0.00%) (0.00%) (0.00%)
Noncontrolling interests     (0.00%)   (0.00%)
Like-Kind Exchange     (3.20%) 0.00%  
Statute of limitations expiration         0.00%
Penalties         0.00%
Merger expenses       0.00% 0.00%
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.00% (2.30%)  
Other     0.30% (0.10%) (1.20%) [4]
Effective income tax rate     1.30% 19.30% 25.40%
Baltimore Gas and Electric Company [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     6.60% 5.40% 5.00% [1]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00%
Amortization of investment tax credit, including deferred taxes on basis difference     (0.10%) (0.10%) (0.10%)
Plant basis differences     (1.30%) [2] 0.10% [2] (2.70%)
Production tax credits and other credits     (0.00%) (0.00%) (0.00%)
Noncontrolling interests     (0.00%)   (0.00%)
Like-Kind Exchange     (8.00%) 0.00%  
Statute of limitations expiration         0.00%
Penalties         0.00%
Merger expenses       0.00% 0.00%
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.00% 0.90%  
Other     0.90% 0.20% 0.00% [4]
Effective income tax rate     19.10% 41.50% 37.20%
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 5  
Potomac Electric Power Company [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00%
State income taxes, net of Federal income tax benefit     2.20% 3.20% 15.70% [1]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00%
Amortization of investment tax credit, including deferred taxes on basis difference     (0.10%) (0.10%) (0.20%)
Plant basis differences     (2.70%) [2] (0.40%) [2] (22.80%)
Production tax credits and other credits     (0.00%) (0.00%) (0.00%)
Noncontrolling interests     (0.00%)   (0.00%)
Like-Kind Exchange     (14.80%) 0.00%  
Statute of limitations expiration         0.00%
Penalties         0.00%
Merger expenses       (6.30%) 23.50%
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.00% 2.70%  
Other     0.20% (0.20%) (1.80%) [4]
Effective income tax rate     5.80% 33.90% 49.40%
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 14  
Delmarva Power & Light Company [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00% [5]
State income taxes, net of Federal income tax benefit     6.70% 5.40% 52.70% [1],[5]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00% [5]
Amortization of investment tax credit, including deferred taxes on basis difference     (0.30%) (0.20%) (3.70%) [5]
Plant basis differences     (0.30%) [2] 2.00% [2] (25.50%) [5]
Production tax credits and other credits     (0.00%) (0.00%) (0.00%) [5]
Noncontrolling interests     (0.00%)   (0.00%) [5]
Like-Kind Exchange     (12.00%) 0.00%  
Statute of limitations expiration [5]         0.00%
Penalties [5]         0.00%
Merger expenses       (7.80%) 112.90% [5]
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.00% 2.50%  
Other     0.40% 0.10% (2.20%) [4],[5]
Effective income tax rate     15.50% 37.00% 169.20% [5]
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 6  
Atlantic City Electric Company [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00% 35.00% [5]
State income taxes, net of Federal income tax benefit     7.40% 5.60% 6.20% [1],[5]
Qualified nuclear decommissioning trust fund income     0.00% 0.00% 0.00% [5]
Amortization of investment tax credit, including deferred taxes on basis difference     (0.40%) (0.40%) 0.80% [5]
Plant basis differences     (0.50%) [2] 3.60% [2] 10.30% [5]
Production tax credits and other credits     (0.00%) (0.00%) (0.00%) [5]
Noncontrolling interests     (0.00%)   (0.00%) [5]
Like-Kind Exchange     (14.90%) 0.00%  
Statute of limitations expiration [5]         0.00%
Penalties [5]         0.00%
Merger expenses       (19.80%) (44.90%) [5]
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.00% 1.60%  
Other     1.20% (0.40%) 1.30% [4],[5]
Effective income tax rate     13.80% 25.20% 8.70% [5]
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 7  
Pepco Holdings LLC [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate     21.00% 35.00%  
State income taxes, net of Federal income tax benefit     3.00% 4.80%  
Qualified nuclear decommissioning trust fund income     0.00% 0.00%  
Amortization of investment tax credit, including deferred taxes on basis difference     (0.20%) (0.20%)  
Plant basis differences [2]     (1.60%) 1.10%  
Production tax credits and other credits     (0.00%) (0.00%)  
Noncontrolling interests     (0.00%)    
Like-Kind Exchange     (14.50%) 0.00%  
Merger expenses       (9.50%)  
FitzPatrick bargain purchase gain       0.00%  
Tax Cuts and Jobs Act of 2017 [3]     0.10% 6.40%  
Other     0.30% (0.10%)  
Effective income tax rate     8.10% 37.50%  
Amount of Impairment to Carrying Amount of Regulatory Assets       $ 27  
Pepco Holdings LLC [Member] | Successor [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate [5]   35.00%      
State income taxes, net of Federal income tax benefit [1],[5]   5.80%      
Qualified nuclear decommissioning trust fund income [5]   0.00%      
Amortization of investment tax credit, including deferred taxes on basis difference [5]   1.40%      
Plant basis differences [5]   39.00%      
Production tax credits and other credits [5]   (0.00%)      
Noncontrolling interests [5]   (0.00%)      
Statute of limitations expiration [5]   0.00%      
Penalties [5]   (0.70%)      
Merger expenses [5]   (89.00%)      
Other [4],[5]   3.30%      
Effective income tax rate [5]   (5.20%)      
Pepco Holdings LLC [Member] | Predecessor [Member]          
Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate [Line Items]          
U.S. Federal statutory rate 35.00%        
State income taxes, net of Federal income tax benefit [1] 11.90%        
Qualified nuclear decommissioning trust fund income 0.00%        
Amortization of investment tax credit, including deferred taxes on basis difference (0.90%)        
Plant basis differences (13.50%)        
Production tax credits and other credits (0.00%)        
Noncontrolling interests (0.00%)        
Statute of limitations expiration 0.00%        
Penalties 0.00%        
Merger expenses 11.10%        
Other [4] 3.60%        
Effective income tax rate 47.20%        
[1] Includes a remeasurement of uncertain state income tax positions for Pepco and DPL.
[2] Includes the charges related to the transmission-related income tax regulatory asset for Exelon, ComEd, BGE, PHI, Pepco, DPL and ACE of $35 million, $3 million, $5 million, $27 million, $14 million, $6 million and $7 million, respectively. See Note 4 - Regulatory Matters for additional information.
[3] Included are impacts for TCJA other than the corporate rate change, including revisions further limiting tax deductions for compensation of certain highest paid executives, the write-off of foreign tax credit carryforwards, and loss of a 2015 domestic production activities deduction due to an NOL carryback.
[4] At PECO, includes a cumulative adjustment related to an anticipated gas repairs tax return accounting method change. The method change request was filed and accepted in 2017. No change to the results recorded as of December 31, 2016
[5] 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.