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v2.4.1.9

Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) (USD $) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Premiums Written: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | $ 2,228,270,000 us-gaap_DirectPremiumsWritten | $ 2,133,793,000 us-gaap_DirectPremiumsWritten | $ 1,955,667,000 us-gaap_DirectPremiumsWritten
Assumed |  |  |  |  |  |  |  |  | 26,306,000 us-gaap_AssumedPremiumsWritten | 43,650,000 us-gaap_AssumedPremiumsWritten | 50,938,000 us-gaap_AssumedPremiumsWritten
Ceded premiums written |  |  |  |  |  |  |  |  | (369,296,000)us-gaap_CededPremiumsWritten | (367,284,000)us-gaap_CededPremiumsWritten | (339,722,000)us-gaap_CededPremiumsWritten
Net |  |  |  |  |  |  |  |  | 1,885,280,000 us-gaap_PremiumsWrittenNet | 1,810,159,000 us-gaap_PremiumsWrittenNet | 1,666,883,000 us-gaap_PremiumsWrittenNet
Premiums Earned: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | 2,183,258,000 us-gaap_DirectPremiumsEarned | 2,048,530,000 us-gaap_DirectPremiumsEarned | 1,873,007,000 us-gaap_DirectPremiumsEarned
Assumed |  |  |  |  |  |  |  |  | 34,653,000 us-gaap_AssumedPremiumsEarned | 44,464,000 us-gaap_AssumedPremiumsEarned | 65,884,000 us-gaap_AssumedPremiumsEarned
Ceded premiums earned |  |  |  |  |  |  |  |  | (365,302,000)us-gaap_CededPremiumsEarned | (356,922,000)us-gaap_CededPremiumsEarned | (354,772,000)us-gaap_CededPremiumsEarned
Total Net Premiums Earned | 469,850,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 462,639,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 463,625,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 456,495,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 451,312,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 437,568,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 426,252,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 420,940,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 1,852,609,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 1,736,072,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 1,584,119,000 us-gaap_PremiumsEarnedNetPropertyAndCasualty
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | 1,314,864,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross | 1,370,293,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross | 2,394,640,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross
Assumed |  |  |  |  |  |  |  |  | 26,187,000 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed | 32,678,000 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed | 29,175,000 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed
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
Net |  |  |  |  |  |  |  |  | 1,157,501,000 us-gaap_IncurredClaimsPropertyCasualtyAndLiability | 1,121,738,000 us-gaap_IncurredClaimsPropertyCasualtyAndLiability | 1,120,990,000 us-gaap_IncurredClaimsPropertyCasualtyAndLiability
Superstorm Sandy [Member] |  |  |  |  |  |  |  |  |  |  |
Increase Decrease In Reinsurance Effect On Claims And Benefits Incurred Amount Ceded |  |  |  |  |  |  |  |  |  |  | $ 1,100,000,000 sigi_IncreaseDecreaseInReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = sigi_SuperstormSandyMember

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- Definition Amount of earned premiums assumed from 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 + Details
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