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Fair Value Measurements - Reconciliation of Beginning and Ending Net Balances for Assets and Liabilities Measured at Level 3 Fair Value (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance $ 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Included in Earnings 30us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Included in Regulatory Assets and Liabilities 7ed_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIncludedInRegulatoryAssetsAndLiabilities 18ed_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIncludedInRegulatoryAssetsAndLiabilities
Purchases 22us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases 17us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Settlements (48)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements (28)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Ending Balance 20us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
CECONY [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ dei_LegalEntityAxis
= ed_CeconyMember
10us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ dei_LegalEntityAxis
= ed_CeconyMember
Included in Earnings 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ dei_LegalEntityAxis
= ed_CeconyMember
7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ dei_LegalEntityAxis
= ed_CeconyMember
Included in Regulatory Assets and Liabilities 7ed_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIncludedInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= ed_CeconyMember
(1)ed_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIncludedInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= ed_CeconyMember
Purchases 16us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ dei_LegalEntityAxis
= ed_CeconyMember
13us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ dei_LegalEntityAxis
= ed_CeconyMember
Settlements (18)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ dei_LegalEntityAxis
= ed_CeconyMember
(23)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ dei_LegalEntityAxis
= ed_CeconyMember
Ending Balance $ 13us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ dei_LegalEntityAxis
= ed_CeconyMember
$ 6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ dei_LegalEntityAxis
= ed_CeconyMember