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Derivative Instruments and Hedging Activities - Additional Information (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2016
USD ($)
Investment Holdings [Line Items]  
Energy supply and hedging activities credit exposure total $ 141
Makeup of net credit exposure with commodity exchange brokers 20
Makeup of net credit exposure independent system operators 77
Makeup of net credit exposure with investment-grade counterparties 34
Makeup of net credit exposure non-investment grade/non-rated counterparties 10
CECONY  
Investment Holdings [Line Items]  
Energy supply and hedging activities credit exposure total 13
Makeup of net credit exposure with commodity exchange brokers 12
Makeup of net credit exposure with investment-grade counterparties $ 1