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

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The carrying value of investments pledged as collateral for reinsurance obligations

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Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 320

-Section 50

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 470

-Section 45

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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

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-Publisher FASB

-Name Accounting Standards Codification

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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.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

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