Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory assets
Regulatory assets - noncurrent	$ 9,156us-gaap_RegulatoryAssetsNoncurrent	$ 7,201us-gaap_RegulatoryAssetsNoncurrent
Regulatory assets - current	148us-gaap_RegulatoryAssetsCurrent	29us-gaap_RegulatoryAssetsCurrent
Total Regulatory Assets	9,304us-gaap_RegulatoryAssets	7,230us-gaap_RegulatoryAssets
Regulatory liabilities
Regulatory liabilities - noncurrent	1,993us-gaap_RegulatoryLiabilityNoncurrent	1,728us-gaap_RegulatoryLiabilityNoncurrent
Regulatory liabilities - current	187us-gaap_RegulatoryLiabilityCurrent	148us-gaap_RegulatoryLiabilityCurrent
Total Regulatory Liabilities	2,180us-gaap_RegulatoryLiabilities	1,876us-gaap_RegulatoryLiabilities
Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	598	540
2014 Rate Plan Rate Base Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	155
Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	138	133
Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	105	40
Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	87	130
Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	78	105
Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	58	88
New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	62
World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	41	62
Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	21	28
Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	19	22
Other [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	290	208
Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	128	100
Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	30	34
Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	24
Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	5	14
Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	4,846	2,730
Future Income Tax [Member]
Regulatory assets
Regulatory assets - noncurrent	2,273	2,145
Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	925	938
Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	319	441
Revenue Taxes [Member]
Regulatory assets
Regulatory assets - noncurrent	219	207
Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - noncurrent	99	78
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	66	237
Regulatory liabilities
Regulatory liabilities - noncurrent	46	50
Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	63	83
Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - noncurrent	57	65
O & R Property Tax Reconciliation [Member]
Regulatory assets
Regulatory assets - noncurrent	36	22
O&R Transition Bond Charges [Member]
Regulatory assets
Regulatory assets - noncurrent	27	33
Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - noncurrent	27	28
Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - noncurrent	25	8
Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - noncurrent	19	29
Regulatory assets - current	41	4
Workers' Compensation [Member]
Regulatory assets
Regulatory assets - noncurrent	8	12
Other [Member]
Regulatory assets
Regulatory assets - noncurrent	147	145
Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	97	25
Future Income Tax - Current [Member]
Regulatory assets
Regulatory assets - current	10
Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	295	322
CECONY [Member]
Regulatory assets
Regulatory assets - noncurrent	8,481	6,639
Regulatory assets - current	132	26
Total Regulatory Assets	8,613	6,665
Regulatory liabilities
Regulatory liabilities - noncurrent	1,837	1,598
Regulatory liabilities - current	142	107
Total Regulatory Liabilities	1,979	1,705
CECONY [Member] \| Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	499	453
CECONY [Member] \| 2014 Rate Plan Rate Base Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	155
CECONY [Member] \| Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	138	133
CECONY [Member] \| Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	105	40
CECONY [Member] \| Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	87	130
CECONY [Member] \| Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	78	105
CECONY [Member] \| Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	57	87
CECONY [Member] \| New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	59
CECONY [Member] \| World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	41	62
CECONY [Member] \| Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	20	20
CECONY [Member] \| Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	18	18
CECONY [Member] \| Other [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	248	178
CECONY [Member] \| Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	84	66
CECONY [Member] \| Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	30	30
CECONY [Member] \| Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	24
CECONY [Member] \| Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	4	11
CECONY [Member] \| Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	4,609	2,610
CECONY [Member] \| Future Income Tax [Member]
Regulatory assets
Regulatory assets - noncurrent	2,166	2,030
CECONY [Member] \| Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	820	830
CECONY [Member] \| Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	224	334
CECONY [Member] \| Revenue Taxes [Member]
Regulatory assets
Regulatory assets - noncurrent	208	196
CECONY [Member] \| Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - noncurrent	92	74
CECONY [Member] \| Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	42	211
Regulatory liabilities
Regulatory liabilities - noncurrent	37	50
CECONY [Member] \| Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	63	83
CECONY [Member] \| Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - noncurrent	55	62
CECONY [Member] \| Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - noncurrent	27	28
CECONY [Member] \| Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - noncurrent	23	7
CECONY [Member] \| Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - noncurrent	17	28
Regulatory assets - current	40	4
CECONY [Member] \| Workers' Compensation [Member]
Regulatory assets
Regulatory assets - noncurrent	8	12
CECONY [Member] \| Other [Member]
Regulatory assets
Regulatory assets - noncurrent	127	134
CECONY [Member] \| Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	92	22
CECONY [Member] \| Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	$ 295	$ 322

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

+ Details

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- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 405

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

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- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.24)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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