Electricity Purchase Agreements - Summary of Estimated Capacity and Other Fixed Payments (Detail) (CECONY [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fixed payment under the contract	$ 1,164	$ 1,372	$ 1,656
2013 [Member]
Fixed payment under the contract	507
2014 [Member]
Fixed payment under the contract	446
2015 [Member]
Fixed payment under the contract	229
2016 [Member]
Fixed payment under the contract	165
2017 [Member]
Fixed payment under the contract	110
All Years Thereafter [Member]
Fixed payment under the contract	$ 933