Electricity Purchase Agreements - Summary of Estimated Capacity and Other Fixed Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
2016
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	$ 222
2017
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	207
2018
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	181
2019
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	98
2020
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	53
All Years Thereafter
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	762
CECONY
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	865	$ 1,312	$ 1,282
CECONY | 2016
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	218
CECONY | 2017
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	205
CECONY | 2018
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	181
CECONY | 2019
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	98
CECONY | 2020
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	53
CECONY | All Years Thereafter
Long-term Contract for Purchase of Electric Power [Line Items]
Fixed payment under the contracts	$ 762