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v2.4.1.9

Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Ceded premiums written | $ (86,776)us-gaap_CededPremiumsWritten | $ (88,991)us-gaap_CededPremiumsWritten
Ceded premiums earned | (91,827)us-gaap_CededPremiumsEarned | (90,390)us-gaap_CededPremiumsEarned
Ceded loss and loss expense incurred | (31,909)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded | (45,282)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
National Flood Insurance Program [Member] |  |
Ceded premiums written | (53,842)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (57,303)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded premiums earned | (58,998)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | (58,286)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded loss and loss expense incurred | $ (6,249)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember | $ (7,379)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember

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- Definition Amount of earned premiums ceded to other entities. + References Reference 1: 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 2: 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 3: 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 4: 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
Name: | us-gaap_CededPremiumsEarned
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of written premiums ceded to other entities. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | sigi_FloodCededPremiumsAndLossesAxis=sigi_NationalFloodInsuranceProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |