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Stock-Based Compensation Plans - Schedule of Stock-based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 23, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense   $ 191 $ 175 [1] $ 119
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense     (18) (3)
Allocated Share-based Compensation Expense, Net of Tax   117 107 [1] 73
Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense   107 93 [1] 41
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense   (29) (18) (18)
Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense   77 75 [1] 71
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense   (35) (27) (30)
Employee Stock Option [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense   0 0 [1] 1
Stock Compensation Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense   7 7 [1] 6
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense   $ (74) $ (68) [1] (46)
Predecessor [Member] | Pepco Holdings LLC [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense $ 3     13
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense (1)     (5)
Allocated Share-based Compensation Expense, Net of Tax 2     8
Predecessor [Member] | Pepco Holdings LLC [Member] | Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense 1     5
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 0     5
Predecessor [Member] | Pepco Holdings LLC [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense 2     7
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 0     2
Predecessor [Member] | Pepco Holdings LLC [Member] | Employee Stock Option [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Allocated Share-based Compensation Expense $ 0     $ 1
[1] (a)2016 amounts include expense related to stock-based compensation granted to eligible PHI employees since the merger date of March 23, 2016.