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Fair Value (Fair Value Measurement Levels For All Assets, Recorded At Fair And Market Value) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value $ 5,226,919cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue $ 5,293,411cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
Fixed maturities - available for sale, at fair value 363us-gaap_AvailableForSaleSecuritiesDebtSecurities 1,509us-gaap_AvailableForSaleSecuritiesDebtSecurities
Equity securities - available for sale, at market value 16cik0000914748_AvailableForSaleSecuritiesEquitySecuritiesMarketValue 16cik0000914748_AvailableForSaleSecuritiesEquitySecuritiesMarketValue
Equity securities - available for sale, at fair value 1,350,070us-gaap_AvailableForSaleSecuritiesEquitySecurities 1,299,037us-gaap_AvailableForSaleSecuritiesEquitySecurities
Other invested assets, at fair value 1,691,275us-gaap_OtherAssetsFairValueDisclosure 1,655,311us-gaap_OtherAssetsFairValueDisclosure
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Equity securities - available for sale, at market value 16cik0000914748_AvailableForSaleSecuritiesEquitySecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 16cik0000914748_AvailableForSaleSecuritiesEquitySecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Equity securities - available for sale, at fair value 1,236,565us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 1,188,613us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Other invested assets, at fair value 1,691,275us-gaap_OtherAssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 1,655,311us-gaap_OtherAssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 5,218,141cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 5,277,648cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Fixed maturities - available for sale, at fair value 363us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 1,509us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Equity securities - available for sale, at fair value 113,505us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 110,424us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 8,778cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 15,763cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 177,523cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember		 136,836cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 177,523cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 136,836cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
State, Municipalities and Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 788,842cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 824,472cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
State, Municipalities and Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 788,842cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 824,472cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Corporate Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 1,971,236cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember		 1,978,935cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
Corporate Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 1,968,583cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 1,978,935cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Corporate Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 2,653cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		  
Asset-Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 116,087cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember		 94,823cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
Asset-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 116,087cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 94,823cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Commercial Mortgage-Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 58,279cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 59,268cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 58,279cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 50,671cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Commercial Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value   8,597cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Agency Residential Mortgage-Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 575,361cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 598,117cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Agency Residential Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 575,361cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 598,117cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Non-Agency Residential Mortgage-Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 277cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 315cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Non-Agency Residential Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 277cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 315cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Foreign Government Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 482,277cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 537,087cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Foreign Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 482,277cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 537,087cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Foreign Corporate Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 1,057,037cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		 1,063,558cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
Foreign Corporate Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value 1,050,912cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 1,056,392cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Foreign Corporate Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturity securities - available for sale, at market value $ 6,125cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 $ 7,166cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member