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Investments (Investments Pledged as Collateral) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		$ 60.7us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Outstanding borrowing with the Federal Home Loan Bank of Indianapolis	45us-gaap_FederalHomeLoanBankAdvancesLongTerm
Securities pledged as collateral for reinsurance obligations		100.4sigi_SecuritiesPledgedAsCollateral
Securities Pledged As Collateral For Reinsurance Obligations		14.4sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations
Assets Held by Insurance Regulators		25.3us-gaap_AssetsHeldByInsuranceRegulators
U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	7.7us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Securities pledged as collateral for reinsurance obligations	33.0sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Securities Pledged As Collateral For Reinsurance Obligations	0sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Assets Held by Insurance Regulators	25.3us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_USTreasuryAndGovernmentMember
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		0us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities pledged as collateral for reinsurance obligations		5.7sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities Pledged As Collateral For Reinsurance Obligations		5.7sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Assets Held by Insurance Regulators		0us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		0us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Securities pledged as collateral for reinsurance obligations		5.3sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations		5.3sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Assets Held by Insurance Regulators		0us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		0us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Securities pledged as collateral for reinsurance obligations		1.1sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations		1.1sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Assets Held by Insurance Regulators		0us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		2.2us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Securities pledged as collateral for reinsurance obligations		2.2sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations		0sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Assets Held by Insurance Regulators		0us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		50.8us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Securities pledged as collateral for reinsurance obligations		53.1sigi_SecuritiesPledgedAsCollateral / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Securities Pledged As Collateral For Reinsurance Obligations		2.3sigi_SecuritiesPledgedAsCollateralForReinsuranceObligations / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Assets Held by Insurance Regulators		$ 0us-gaap_AssetsHeldByInsuranceRegulators / invest_InvestmentSectorAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember

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Total value amount of investments pledged as collateral

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The carrying value of investments pledged as collateral for reinsurance obligations

+ References

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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

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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



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-Section 50

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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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 470

-Section 45

-Paragraph 1

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-Paragraph 1

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-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9



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