Goodwill - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line Items]
Goodwill	$ 429	$ 429
O&R [Member]
Goodwill [Line Items]
Goodwill	406	406
CECONY [Member]
Goodwill [Line Items]
Goodwill	245	245
O&R [Member]
Goodwill [Line Items]
Goodwill	161	161
Energy Services Acquired [Member]
Goodwill [Line Items]
Goodwill	$ 23	$ 23