Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Con Edison [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 299	$ 279
Total liabilities	53	78
CECONY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	243	227
Total liabilities	25	39
Commodity [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	62	66
Commodity [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	50	72
Commodity [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	23	30
Commodity [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	25	39
Other assets [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	237	213
Other assets [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	220	197
Interest rate contract [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	3	6
Interest rate contract [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 1 [Member] | Con Edison [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	130	106
Total liabilities	9	12
Level 1 [Member] | CECONY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	123	99
Total liabilities	9	12
Level 1 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1
Level 1 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	9	12
Level 1 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1
Level 1 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	9	12
Level 1 [Member] | Other assets [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	129	106
Level 1 [Member] | Other assets [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	122	99
Level 1 [Member] | Interest rate contract [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 1 [Member] | Interest rate contract [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 2 [Member] | Con Edison [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	160	150
Total liabilities	77	122
Level 2 [Member] | CECONY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	104	106
Total liabilities	37	62
Level 2 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	52	43
Level 2 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	74	116
Level 2 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	6	8
Level 2 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	37	62
Level 2 [Member] | Other assets [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	108	107
Level 2 [Member] | Other assets [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	98	98
Level 2 [Member] | Interest rate contract [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	3	6
Level 2 [Member] | Interest rate contract [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 3 [Member] | Con Edison [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	9	33
Total liabilities	23	38
Level 3 [Member] | CECONY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	8	10
Total liabilities
Level 3 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	9	33
Level 3 [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	23	38
Level 3 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	8	10
Level 3 [Member] | Commodity [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 3 [Member] | Other assets [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets
Level 3 [Member] | Other assets [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets
Level 3 [Member] | Interest rate contract [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Level 3 [Member] | Interest rate contract [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Netting Adjustments [Member] | Con Edison [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		(10)
Total liabilities	(56)	(94)
Netting Adjustments [Member] | CECONY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	8	12
Total liabilities	(21)	(35)
Netting Adjustments [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		(10)
Netting Adjustments [Member] | Commodity [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(56)	(94)
Netting Adjustments [Member] | Commodity [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	8	12
Netting Adjustments [Member] | Commodity [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	(21)	(35)
Netting Adjustments [Member] | Other assets [Member] | Con Edison [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets
Netting Adjustments [Member] | Other assets [Member] | CECONY [Member] | Derivatives Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets
Netting Adjustments [Member] | Interest rate contract [Member] | Con Edison [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities
Netting Adjustments [Member] | Interest rate contract [Member] | CECONY [Member] | Derivatives Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities