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Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2013
Manufactured gas plant sites [Member]
Mar. 31, 2012
Manufactured gas plant sites [Member]
Mar. 31, 2013
Other Superfund Sites [Member]
Mar. 31, 2012
Other Superfund Sites [Member]
Mar. 31, 2013
Superfund Sites [Member]
Mar. 31, 2012
Superfund Sites [Member]
Mar. 31, 2013
CECONY [Member]
Dec. 31, 2012
CECONY [Member]
Mar. 31, 2013
CECONY [Member]
Manufactured gas plant sites [Member]
Mar. 31, 2012
CECONY [Member]
Manufactured gas plant sites [Member]
Mar. 31, 2013
CECONY [Member]
Other Superfund Sites [Member]
Mar. 31, 2012
CECONY [Member]
Other Superfund Sites [Member]
Mar. 31, 2013
CECONY [Member]
Superfund Sites [Member]
Mar. 31, 2012
CECONY [Member]
Superfund Sites [Member]
Accrued Liabilities:                                
Accrued Liabilities     $ 457 $ 462 $ 79 $ 83 $ 536 $ 545     $ 348 $ 351 $ 77 $ 82 $ 425 $ 433
Regulatory assets $ 9,516 $ 9,779         $ 721 $ 730 $ 8,802 $ 9,032         $ 607 $ 615