Income Tax - Summary of Unrecognized Tax Benefits (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Reconciliation [Line Items]
Balance at January 1	$ 130	$ 93	$ 86
Additions based on tax positions related to the current year	12	76	5
Additions based on tax positions of prior years		4	67
Reductions for tax positions of prior years	(57)	(43)	(4)
Settlements	1		(61)
Balance at December 31	86	130	93
CECONY [Member]
Income Tax Reconciliation [Line Items]
Balance at January 1	114	79	92
Additions based on tax positions related to the current year	11	74	4
Additions based on tax positions of prior years		3	49
Reductions for tax positions of prior years	(52)	(42)	(4)
Settlements	1		(62)
Balance at December 31	$ 74	$ 114	$ 79