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Investments (Investments Pledged as Collateral) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | $ 64.0 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Securities Pledged As Collateral For Reinsurance Obligations | 13.0 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 25.2 us-gaap_AssetsHeldByInsuranceRegulators
Securities Pledged As Collateral | 102.2 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
Outstanding Borrowing With The Federal Home Loan Bank Of Indianapolis | 60 us-gaap_FederalHomeLoanBankAdvancesLongTerm
U.S. Government and Government Agencies [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 7.7 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Securities Pledged As Collateral For Reinsurance Obligations | 0 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 25.2 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Securities Pledged As Collateral | 32.9 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Obligations of States and Political Subdivisions [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 0 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities Pledged As Collateral For Reinsurance Obligations | 5.0 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 0 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities Pledged As Collateral | 5.0 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 0 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations | 4.8 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 0 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Securities Pledged As Collateral | 4.8 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-Backed Securities [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 0 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations | 1.0 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 0 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Securities Pledged As Collateral | 1.0 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 1.7 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations | 0 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 0 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Securities Pledged As Collateral | 1.7 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Residential Mortgage Backed Securities [Member] |
Investments Pledged As Collateral [Line Items] |
Fixed Income Securities Pledged For FHLBI Collateral | 54.6 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations | 2.2 sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies | 0 us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Securities Pledged As Collateral | $ 56.8 us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember

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- Definition The carrying value of investments pledged as collateral for reinsurance obligations + References No definition available. + Details
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- Definition Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495 + Details
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- Definition Amount of the collateral pledged in support of federal home loan bank advances outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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- Definition Including both current and noncurrent portions, Federal Home Loan Bank borrowings initially due beyond one year or beyond the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64702-112823 + Details
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- Definition Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6467456&loc=d3e113888-111728 + Details
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