Derivative Instruments and Hedging Activities - Number of Derivative Contracts by Commodity Type (Detail)	3 Months Ended
	Mar. 31, 2013 MWh MW Contract
Electric Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Energy Contracts	493
MWHs	15,267,685
Number of Capacity Contracts	70
MWs	10,750
Electric Derivatives [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Number of Energy Contracts	101
MWHs	3,844,000
Number of Capacity Contracts
MWs
Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Energy Contracts	533
Dths	79,064,410
Number of Capacity Contracts	1,096
Gas Derivatives [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Number of Energy Contracts	452
Dths	74,340,000
Number of Capacity Contracts	553