Electricity Purchase Agreements - Summary of Capacity, Energy 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]
Capacity, energy and other fixed payments	$ 1,312	$ 1,282	$ 1,164
Brooklyn Navy Yard [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	133	118	93
Linden Cogeneration [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	381	346	297
Indeck Corinth [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	80	79	66
Indian Point [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	247	220	204
Astoria Energy [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	230	183	181
Selkirk [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	144	215	196
Independence [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Capacity, energy and other fixed payments	$ 97	$ 121	$ 127