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Fair Value of Financial Assets and Liabilities - Fair Value Reconciliation of Level 3 Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance $ 907 $ 966
Total realized / unrealized gains (losses)    
Included in net income (23) (101)
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities (18) 6
Change In Collateral 138 (5)
Purchases, sales, issuances and settlements    
Purchases 176 226
Sales (23) (4)
Settlements (89) (123)
Transfers into Level 3 5 (22)
Transfers out of Level 3 (5) (36)
Ending Balance 1,068 907
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 359 160
Fair Value, Inputs, Level 3 [Member] | Consolidation, Eliminations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 0 0
Total realized / unrealized gains (losses)    
Included in net income 0 0
Included in noncurrent payables to affiliates (34) 1
Included in regulatory assets/liabilities 34 (1)
Change In Collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 0 0
Sales 0 0
Settlements 0 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 0 0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0 0
Pepco Holdings LLC [Member] | Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 38 22
Total realized / unrealized gains (losses)    
Included in net income (3) (4)
Included in regulatory assets/liabilities 0 0
Change In Collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 0 0
Sales 0 0
Settlements 0 (12)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 41 38
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 3 0
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 1,118 1,200
Total realized / unrealized gains (losses)    
Included in net income (26) (105)
Included in noncurrent payables to affiliates 34 (1)
Included in regulatory assets/liabilities 0 0
Change In Collateral 138 (5)
Purchases, sales, issuances and settlements    
Purchases 176 226
Sales (23) (4)
Settlements (89) (135)
Transfers into Level 3 5 (22)
Transfers out of Level 3 (5) (36)
Ending Balance 1,328 1,118
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 356 160
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 543 648
Total realized / unrealized gains (losses)    
Included in net income 5 0
Included in noncurrent payables to affiliates 34 (1)
Included in regulatory assets/liabilities 0 0
Change In Collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 44 36
Sales (21) 0
Settlements (94) (140)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 511 543
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 5 (5)
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 575 552
Total realized / unrealized gains (losses)    
Included in net income (31) (105)
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities 0 0
Change In Collateral 138 (5)
Purchases, sales, issuances and settlements    
Purchases 132 190
Sales (2) (4)
Settlements 5 5
Transfers into Level 3 5 (22)
Transfers out of Level 3 (5) (36)
Ending Balance 817 575
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 351 165
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (377) (265)
Commonwealth Edison Co [Member]    
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]    
Realized Investment Gains (Losses) 26 17
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member]    
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Period Increase (Decrease) (78) (24)
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance (249) (256)
Total realized / unrealized gains (losses)    
Included in net income 0 0
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities (52) 7
Change In Collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 0 0
Sales 0 0
Settlements 0 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance (301) (249)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0 0
Potomac Electric Power Company [Member] | Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]    
Beginning Balance 38 22
Total realized / unrealized gains (losses)    
Included in net income 3 4
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities 0 0
Change In Collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 0 0
Sales 0 0
Settlements 0 12
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 41 38
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) $ 3 $ 0