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Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Percentage of unpaid NFIP claims ceded by federal government	100.00%sigi_PercentageOfUnpaidNfipClaimsCededByFederalGovernment
Ceded premiums written	$ (369,296,000)us-gaap_CededPremiumsWritten	$ (367,284,000)us-gaap_CededPremiumsWritten	$ (339,722,000)us-gaap_CededPremiumsWritten
Ceded premiums earned	(365,302,000)us-gaap_CededPremiumsEarned	(356,922,000)us-gaap_CededPremiumsEarned	(354,772,000)us-gaap_CededPremiumsEarned
Ceded losses and loss expenses incurred	(183,550,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	(281,233,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	(1,302,825,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
National Flood Insurance Program [Member]
Ceded premiums written	(237,718,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(236,309,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(221,094,000)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded premiums earned	(234,224,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(228,650,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(212,177,000)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded losses and loss expenses incurred	(57,323,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(183,142,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(1,119,303,000)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Superstorm Sandy [Member]
Flood losses ceded to federal government related to Superstorm Sandy			$ 1,100,000,000sigi_IncreaseDecreaseInReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis = sigi_SuperstormSandyMember

X

- Definition

The increase or decrease in the amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

No definition available.

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- Definition

Percentage of unpaid NFIP claims ceded by federal government

+ References

No definition available.

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X

- Definition

Amount of earned premiums ceded to other entities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12



Reference 2: 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



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-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 4: 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 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

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-Section 45

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-URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546

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- Definition

Amount of written premiums ceded to other entities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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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

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