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Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance $ 5 $ 13 $ 6 $ 20
Included in earnings (4) (4) (1) (18)
Included in regulatory assets and liabilities (5) (1) (11) (1)
Purchases 0 1 2 9
Sales 4 0 4 0
Settlements 1 (5) 1 (6)
Ending balance 1 4 1 4
CECONY        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Beginning balance 2 11 8 13
Included in earnings 0 (1) (1) (5)
Included in regulatory assets and liabilities (3) (1) (6) (1)
Purchases 0 1 1 5
Sales 0 0 0 0
Settlements 1 (2) (2) (4)
Ending balance $ 0 $ 8 $ 0 $ 8