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Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning Balance   $ 1
Beginning Balance $ (13)  
Included in earnings (5) 4
Included in regulatory assets and liabilities 18 (10)
Settlements 8 (6)
Transfer out of level 3 (24) (2)
Ending Balance (16) (13)
CECONY    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning Balance   4
Beginning Balance (2)  
Included in earnings 0 4
Included in regulatory assets and liabilities 17 (4)
Settlements 1 (4)
Transfer out of level 3 (22) (2)
Ending Balance $ (6) $ (2)