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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, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning Balance $ 1 $ 6
Included in earnings 8 (7)
Included in regulatory assets and liabilities (13) (6)
Purchases 2 4
Sales 0 4
Settlements (8) 0
Transfer out of level 3 11 0
Ending Balance 1 1
CECONY    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning Balance 1 8
Included in earnings 2 (1)
Included in regulatory assets and liabilities (7) (6)
Purchases 1 2
Sales 0 0
Settlements (3) (2)
Transfer out of level 3 10 0
Ending Balance $ 4 $ 1