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v2.4.1.9

Related Party Transactions (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Equity ownership percentage of director of parent of general independent retail insurance agency | 10.00%sigi_EquityOwnershipPercentageOfDirectorOfParent |  |
Direct premiums written | $ 2,228,270,000 us-gaap_DirectPremiumsWritten | $ 2,133,793,000 us-gaap_DirectPremiumsWritten | $ 1,955,667,000 us-gaap_DirectPremiumsWritten
Premiums paid to Rue Insurance for insurance coverage | 0 sigi_RelatedPartyTransactionsPaymentOfPremiums | 0 sigi_RelatedPartyTransactionsPaymentOfPremiums | 200,000 sigi_RelatedPartyTransactionsPaymentOfPremiums
Contributions to the Selective Group Foundation | 800,000 sigi_RelatedPartyTransactionsContributionsToFoundation | 400,000 sigi_RelatedPartyTransactionsContributionsToFoundation | 400,000 sigi_RelatedPartyTransactionsContributionsToFoundation
Related Party Insurance Agency [Member] |  |  |
Direct premiums written | 9,000,000 us-gaap_DirectPremiumsWritten / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember | 8,200,000 us-gaap_DirectPremiumsWritten / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember | 7,700,000 us-gaap_DirectPremiumsWritten / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember
Commissions paid to Rue Insurance | $ 1,600,000 us-gaap_PaymentsForCommissions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember | $ 1,300,000 us-gaap_PaymentsForCommissions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember | $ 1,300,000 us-gaap_PaymentsForCommissions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = sigi_RelatedPartyInsuranceAgencyMember

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- Definition The minimum percentage of the equity of an independent insurance agency owned by a director of the Parent. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The amount of contributions made to a related party private foundation. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The amount of premiums paid on insurance placed by a related party. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_DirectPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The amount of cash paid for commissions during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |