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v2.4.1.9

Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Premiums written: |  |
Direct | $ 598,783 us-gaap_DirectPremiumsWritten | $ 557,891 us-gaap_DirectPremiumsWritten
Assumed | 6,081 us-gaap_AssumedPremiumsWritten | 7,850 us-gaap_AssumedPremiumsWritten
Ceded premiums written | (86,776)us-gaap_CededPremiumsWritten | (88,991)us-gaap_CededPremiumsWritten
Net | 518,088 us-gaap_PremiumsWrittenNet | 476,750 us-gaap_PremiumsWrittenNet
Premiums earned: |  |
Direct | 562,042 us-gaap_DirectPremiumsEarned | 536,700 us-gaap_DirectPremiumsEarned
Assumed | 5,908 us-gaap_AssumedPremiumsEarned | 10,185 us-gaap_AssumedPremiumsEarned
Ceded premiums earned | (91,827)us-gaap_CededPremiumsEarned | (90,390)us-gaap_CededPremiumsEarned
Net | 476,123 us-gaap_PremiumsEarnedNetPropertyAndCasualty | 456,495 us-gaap_PremiumsEarnedNetPropertyAndCasualty
Loss and loss expense incurred: |  |
Direct | 312,246 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross | 358,349 us-gaap_PolicyholderBenefitsAndClaimsIncurredGross
Assumed | 4,662 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed | 7,479 us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed
Ceded loss and loss expense incurred | (31,909)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded | (45,282)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
Net | $ 284,999 us-gaap_IncurredClaimsPropertyCasualtyAndLiability | $ 320,546 us-gaap_IncurredClaimsPropertyCasualtyAndLiability
Percentage of NFIP claims ceded by federal government | 100.00%sigi_PercentageOfUnpaidNfipClaimsCededByFederalGovernment |

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- Definition Amount of earned premiums assumed from 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 + Details
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- Definition Amount of written premiums assumed from 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)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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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
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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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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + 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)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount, after effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred for property and casualty insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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