Regulatory Matters - Regulatory Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulatory assets
Regulatory assets - noncurrent	$ 8,906us-gaap_RegulatoryAssetsNoncurrent	$ 9,156us-gaap_RegulatoryAssetsNoncurrent
Regulatory assets - current	66us-gaap_RegulatoryAssetsCurrent	148us-gaap_RegulatoryAssetsCurrent
Total Regulatory Assets	8,972us-gaap_RegulatoryAssets	9,304us-gaap_RegulatoryAssets
Regulatory liabilities
Regulatory liabilities - noncurrent	1,892us-gaap_RegulatoryLiabilityNoncurrent	1,993us-gaap_RegulatoryLiabilityNoncurrent
Regulatory liabilities - current	240us-gaap_RegulatoryLiabilityCurrent	187us-gaap_RegulatoryLiabilityCurrent
Total Regulatory Liabilities	2,132us-gaap_RegulatoryLiabilities	2,180us-gaap_RegulatoryLiabilities
Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	4,593	4,846
Future Income Tax [Member]
Regulatory assets
Regulatory assets - noncurrent	2,289	2,273
Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	911	925
Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	287	319
Revenue Taxes [Member]
Regulatory assets
Regulatory assets - noncurrent	223	219
Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - noncurrent	96	99
Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	61	66
Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	58	63
Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - noncurrent	56	57
Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - noncurrent	44	25
Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - noncurrent	42	19
Regulatory assets - current	1	41
O & R Property Tax Reconciliation [Member]
Regulatory assets
Regulatory assets - noncurrent	40	36
O&R Transition Bond Charges [Member]
Regulatory assets
Regulatory assets - noncurrent	25	27
Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - noncurrent	27	27
Workers' Compensation [Member]
Regulatory assets
Regulatory assets - noncurrent	9	8
Other [Member]
Regulatory assets
Regulatory assets - noncurrent	145	147
Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	55	97
Future Income Tax - Current [Member]
Regulatory assets
Regulatory assets - current	10	10
Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	614	598
Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	297	295
2014 Rate Plan Rate Base Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	153	155
Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	104	105
Pension and Other Post Retirement Benefits Deferrals, Liabilities [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	85	46
Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	76	87
Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	74	78
Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	71	138
New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	63	62
Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	52	58
World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	36	41
Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	21	21
Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	19	19
Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	15
Other [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	212	290
Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	177	128
Revenue Decoupling Mechanism [Member]
Regulatory liabilities
Regulatory liabilities - current	30	30
Future Income Tax [Member]
Regulatory liabilities
Regulatory liabilities - current	25	24
Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	8	5
CECONY [Member]
Regulatory assets
Regulatory assets - noncurrent	8,251	8,481
Regulatory assets - current	51	132
Total Regulatory Assets	8,302	8,613
Regulatory liabilities
Regulatory liabilities - noncurrent	1,728	1,837
Regulatory liabilities - current	212	142
Total Regulatory Liabilities	1,940	1,979
CECONY [Member] \| Unrecognized Pension and Other Postretirement Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	4,374	4,609
CECONY [Member] \| Future Income Tax [Member]
Regulatory assets
Regulatory assets - noncurrent	2,180	2,166
CECONY [Member] \| Environmental Remediation Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	808	820
CECONY [Member] \| Deferred Storm Costs [Member]
Regulatory assets
Regulatory assets - noncurrent	196	224
CECONY [Member] \| Revenue Taxes [Member]
Regulatory assets
Regulatory assets - noncurrent	211	208
CECONY [Member] \| Surcharge for New York State Assessment [Member]
Regulatory assets
Regulatory assets - noncurrent	90	92
CECONY [Member] \| Pension and Other Postretirement Benefits Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	35	42
CECONY [Member] \| Net Electric Deferrals [Member]
Regulatory assets
Regulatory assets - noncurrent	58	63
CECONY [Member] \| Unamortized Loss on Reacquired Debt [Member]
Regulatory assets
Regulatory assets - noncurrent	53	55
CECONY [Member] \| Deferred Derivative Losses - Noncurrent [Member]
Regulatory assets
Regulatory assets - noncurrent	40	23
CECONY [Member] \| Recoverable Energy Costs - Current [Member]
Regulatory assets
Regulatory assets - noncurrent	42	17
Regulatory assets - current		40
CECONY [Member] \| Preferred Stock Redemption [Member]
Regulatory assets
Regulatory assets - noncurrent	27	27
CECONY [Member] \| Workers' Compensation [Member]
Regulatory assets
Regulatory assets - noncurrent	9	8
CECONY [Member] \| Other [Member]
Regulatory assets
Regulatory assets - noncurrent	128	127
CECONY [Member] \| Deferred Derivative Losses - Current [Member]
Regulatory assets
Regulatory assets - current	51	92
CECONY [Member] \| Allowance for Cost of Removal Less Salvage [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	514	499
CECONY [Member] \| Property Tax Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	297	295
CECONY [Member] \| 2014 Rate Plan Rate Base Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	153	155
CECONY [Member] \| Prudence Proceeding [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	104	105
CECONY [Member] \| Pension and Other Post Retirement Benefits Deferrals, Liabilities [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	64	37
CECONY [Member] \| Property Tax Refunds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	76	87
CECONY [Member] \| Long-Term Interest Rate Reconciliation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	74	78
CECONY [Member] \| Net Unbilled Revenue Deferrals [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	71	138
CECONY [Member] \| New York State Income Tax Rate Change [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	60	59
CECONY [Member] \| Carrying Charges on Repair Allowance and Bonus Depreciation [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	50	57
CECONY [Member] \| World Trade Center Settlement Proceeds [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	36	41
CECONY [Member] \| Carrying Charges on Transmission and Distribution Net Plant [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	21	20
CECONY [Member] \| Electric Excess Earnings [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	18	18
CECONY [Member] \| Unrecognized Other Postretirement Benefits Costs [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	13
CECONY [Member] \| Other [Member]
Regulatory liabilities
Regulatory liabilities - noncurrent	177	248
CECONY [Member] \| Refundable Energy Costs - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	152	84
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	24
CECONY [Member] \| Deferred Derivative Gains - Current [Member]
Regulatory liabilities
Regulatory liabilities - current	$ 6	$ 4

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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

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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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

+ Details

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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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