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Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Detail) (Other Postretirement Benefits [Member], USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]        
Total investments $ 725ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments $ 743ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments    
Funds for retiree health benefits 358us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare 372us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare    
Investments (including funds for retiree health benefits) 1,083ed_InvestmentsIncludingFundsForRetireeHealthBenefits 1,115ed_InvestmentsIncludingFundsForRetireeHealthBenefits    
Pending activities 1ed_DefinedBenefitPlanPendingActivities (2)ed_DefinedBenefitPlanPendingActivities    
Total fair value of plan net assets 1,084us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 1,113us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 1,047us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 947us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Equity [Member]
       
Defined Benefit Plan Disclosure [Line Items]        
Total investments 428ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
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450ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
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= us-gaap_EquitySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Other Fixed Income Debt [Member]
       
Defined Benefit Plan Disclosure [Line Items]        
Total investments 286ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= ed_OtherFixedIncomeMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
286ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= ed_OtherFixedIncomeMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Cash and Cash Equivalents [Member]
       
Defined Benefit Plan Disclosure [Line Items]        
Total investments 11ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
7ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Level 1 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Funds for retiree health benefits 184us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
185us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
   
Investments (including funds for retiree health benefits) 184ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
185ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
   
Level 2 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 725ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
743ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Funds for retiree health benefits 131us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
145us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Investments (including funds for retiree health benefits) 856ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
888ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Level 2 [Member] | Equity [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 428ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
450ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Level 2 [Member] | Other Fixed Income Debt [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 286ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= ed_OtherFixedIncomeMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
286ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= ed_OtherFixedIncomeMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Level 2 [Member] | Cash and Cash Equivalents [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total investments 11ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
7ed_DefinedBenefitPlanFairValueOfPlanAssetsInvestments
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
   
Level 3 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Funds for retiree health benefits 43us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
42us-gaap_SupplementalUnemploymentBenefitsContinuationOfHealthCare
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
   
Investments (including funds for retiree health benefits) $ 43ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
$ 42ed_InvestmentsIncludingFundsForRetireeHealthBenefits
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member