Income Tax - Summary of Unrecognized Tax Benefits (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Contingency [Line Items]
Balance at January 1		$ 86	$ 130	$ 93
Additions based on tax positions related to the current year		5	12	76
Additions based on tax positions of prior years		253		4
Reductions for tax positions of prior years	(249)	(86)	(57)	(43)
Settlements		(249)	1
Balance at December 31		9	86	130
CECONY [Member]
Income Tax Contingency [Line Items]
Balance at January 1		74	114	79
Additions based on tax positions related to the current year			11	74
Additions based on tax positions of prior years				3
Reductions for tax positions of prior years		(74)	(52)	(42)
Settlements			1
Balance at December 31			$ 74	$ 114