Fair Value (Narrative) (Details) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of public equity portfolio	$ 169,165	$ 169,165	$ 133,755
Transfers between Level 1 and Level 2	0	0
Transfers from level 3
Investments within other invested assets	1,724,532	1,724,532	1,766,626
Available For Sale Securities Debt Securities Market Value	6,140,028	6,140,028	5,970,496
NAV Practical Expedient Guidance ASU 2015-07
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments within other invested assets	52,139	$ 52,139	$ 18,801
Private Placement [Member] \| Unavailability Of Prices [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of securities \| security		46	42
Available For Sale Securities Debt Securities Market Value	$ 89,291	$ 89,291	$ 86,536

Amount of debt securities categorized neither as held-to-maturity nor trading, held at market value.

No definition available.

Name:	cik0000914748_AvailableForSaleSecuritiesDebtSecuritiesMarketValue
Namespace Prefix:	cik0000914748_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number Of Securities

+ References

No definition available.

+ Details

Name:	cik0000914748_NumberOfSecurities
Namespace Prefix:	cik0000914748_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Public equity portfolio fair value disclosure.

+ References

No definition available.

+ Details

Name:	cik0000914748_PublicEquityPortfolioFairValueDisclosure
Namespace Prefix:	cik0000914748_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value portion of other assets.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherAssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_NewAccountingPronouncementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_PrivatePlacementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: