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, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	$ 1,312	$ 1,282	$ 1,164
2015 [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	252
2016 [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	186
2017 [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	135
Year 2018 [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	62
2019 [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	53
All Years Thereafter [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	$ 820