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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, 2020
Dec. 31, 2019
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 1,068 $ 907
Total realized / unrealized gains (losses)    
Included in net income (409) (23)
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities 21 (18)
Change in collateral 53 (138)
Purchases, sales, issuances and settlements    
Purchases 151 176
Sales (27) (23)
Settlements (55) (89)
Transfers into Level 3 (12) 5
Transfers out of Level 3 (24) (5)
Ending Balance 660 1,068
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 11 359
Fair Value, Inputs, Level 3 [Member] | Consolidation, Eliminations [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 0 0
Total realized / unrealized gains (losses)    
Included in net income 0 0
Included in noncurrent payables to affiliates (21) (34)
Included in regulatory assets/liabilities 21 34
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
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 1,328 1,118
Total realized / unrealized gains (losses)    
Included in net income (412) (26)
Included in noncurrent payables to affiliates 21 34
Included in regulatory assets/liabilities 0 0
Change in collateral 53 (138)
Purchases, sales, issuances and settlements    
Purchases 151 176
Sales (27) (23)
Settlements (45) (89)
Transfers into Level 3 (12) 5
Transfers out of Level 3 (24) (5)
Ending Balance 927 1,328
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 8 356
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Trust Fund Investments [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 511 543
Total realized / unrealized gains (losses)    
Included in net income 2 5
Included in noncurrent payables to affiliates 21 34
Included in regulatory assets/liabilities 0 0
Change in collateral 0 0
Purchases, sales, issuances and settlements    
Purchases 8 44
Sales 0 (21)
Settlements (45) (94)
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 497 511
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 2 5
Exelon Generation Co L L C [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 817 575
Total realized / unrealized gains (losses)    
Included in net income (414) (31)
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities 0 0
Change in collateral 53 (138)
Purchases, sales, issuances and settlements    
Purchases 143 132
Sales (27) (2)
Settlements 0 5
Transfers into Level 3 (12) 5
Transfers out of Level 3 (24) (5)
Ending Balance 430 817
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 6 351
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings 420 377
Commonwealth Edison Co [Member] | Derivative [Member] | Interest Rate Swap [Member]    
Purchases, sales, issuances and settlements    
Realized Investment Gains (Losses) 33 26
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance (301) (249)
Total realized / unrealized gains (losses)    
Included in net income 0 0
Included in noncurrent payables to affiliates 0 0
Included in regulatory assets/liabilities 0 (52)
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) (301)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0 0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Period Increase (Decrease) 33 78
Pepco Holdings LLC [Member] | 6311 Life Insurance [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 41 38
Total realized / unrealized gains (losses)    
Included in net income 3 3
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 (10) 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 34 41
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 3 3
Potomac Electric Power Company [Member] | 6311 Life Insurance [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 41 38
Total realized / unrealized gains (losses)    
Included in net income 3 3
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 (10) 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending Balance 34 41
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) $ 3 $ 3