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Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Percentage of unpaid NFIP claims ceded by federal government | 100.00%sigi_PercentageOfUnpaidNfipClaimsCededByFederalGovernment |  |
Ceded premiums written | $ (369,296,000)us-gaap_CededPremiumsWritten | $ (367,284,000)us-gaap_CededPremiumsWritten | $ (339,722,000)us-gaap_CededPremiumsWritten
Ceded premiums earned | (365,302,000)us-gaap_CededPremiumsEarned | (356,922,000)us-gaap_CededPremiumsEarned | (354,772,000)us-gaap_CededPremiumsEarned
Ceded losses and loss expenses incurred | (183,550,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded | (281,233,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded | (1,302,825,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
National Flood Insurance Program [Member] |  |  |
Ceded premiums written | (237,718,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (236,309,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (221,094,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded premiums earned | (234,224,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (228,650,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (212,177,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded losses and loss expenses incurred | (57,323,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (183,142,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (1,119,303,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Superstorm Sandy [Member] |  |  |
Flood losses ceded to federal government related to Superstorm Sandy |  |  | $ 1,100,000,000 sigi_IncreaseDecreaseInReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = sigi_SuperstormSandyMember

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- Definition The increase or decrease in the amount of reduction of provision for policy benefits and costs incurred for policies ceded. + 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 Percentage of unpaid NFIP claims ceded by federal government + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Amount of earned premiums ceded to other entities. + 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 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546 + Details
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- Definition Amount of written premiums ceded to other entities. + 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)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount of reduction of provision for policy benefits and costs incurred for policies ceded. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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- Details
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- Details
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