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Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Ceded premiums written	$ (86,776)us-gaap_CededPremiumsWritten	$ (88,991)us-gaap_CededPremiumsWritten
Ceded premiums earned	(91,827)us-gaap_CededPremiumsEarned	(90,390)us-gaap_CededPremiumsEarned
Ceded loss and loss expense incurred	(31,909)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	(45,282)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
National Flood Insurance Program [Member]
Ceded premiums written	(53,842)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(57,303)us-gaap_CededPremiumsWritten / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded premiums earned	(58,998)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	(58,286)us-gaap_CededPremiumsEarned / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember
Ceded loss and loss expense incurred	$ (6,249)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember	$ (7,379)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded / sigi_FloodCededPremiumsAndLossesAxis = sigi_NationalFloodInsuranceProgramMember

X

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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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Balance Type:	debit
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X

- 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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Namespace Prefix:
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