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Fair Value Measurements - Reconciliation of Assets and Liabilities Measured at Level 3 Fair Value (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ (9) $ (14) $ (19) $ (16)
Included in earnings 21 0 20 (7)
Included in regulatory assets and liabilities (17) (3) (9) (3)
Settlements 0 0 3 9
Ending balance (5) (17) (5) (17)
CECONY | Subsidiaries        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance (7) (7) (10) (6)
Included in earnings 1 0 (2) (3)
Included in regulatory assets and liabilities (11) (1) (7) (3)
Settlements 0 (1) 2 3
Ending balance $ (17) $ (9) $ (17) $ (9)