Derivative Instruments and Hedging Activities - Number of Derivative Contracts by Commodity Type (Detail)	12 Months Ended
	Dec. 31, 2012 MWh MW Contract
Electric Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	496
Mwhs	15,761,464
Number of Capacity Contracts	59
MWs	11,668
Electric Derivatives [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	97
Mwhs	3,565,325
Number of Capacity Contracts
MWs
Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	592
Dths	84,706,809
Number of Capacity Contracts	1,147
Gas Derivatives [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	484
Dths	79,460,000
Number of Capacity Contracts	581