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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Premiums written:
Direct	$ 598,783us-gaap_DirectPremiumsWritten	$ 557,891us-gaap_DirectPremiumsWritten
Assumed	6,081us-gaap_AssumedPremiumsWritten	7,850us-gaap_AssumedPremiumsWritten
Ceded premiums written	(86,776)us-gaap_CededPremiumsWritten	(88,991)us-gaap_CededPremiumsWritten
Net	518,088us-gaap_PremiumsWrittenNet	476,750us-gaap_PremiumsWrittenNet
Premiums earned:
Direct	562,042us-gaap_DirectPremiumsEarned	536,700us-gaap_DirectPremiumsEarned
Assumed	5,908us-gaap_AssumedPremiumsEarned	10,185us-gaap_AssumedPremiumsEarned
Ceded premiums earned	(91,827)us-gaap_CededPremiumsEarned	(90,390)us-gaap_CededPremiumsEarned
Net	476,123us-gaap_PremiumsEarnedNetPropertyAndCasualty	456,495us-gaap_PremiumsEarnedNetPropertyAndCasualty
Loss and loss expense incurred:
Direct	312,246us-gaap_PolicyholderBenefitsAndClaimsIncurredGross	358,349us-gaap_PolicyholderBenefitsAndClaimsIncurredGross
Assumed	4,662us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed	7,479us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountAssumed
Ceded loss and loss expense incurred	(31,909)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded	(45,282)us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
Net	$ 284,999us-gaap_IncurredClaimsPropertyCasualtyAndLiability	$ 320,546us-gaap_IncurredClaimsPropertyCasualtyAndLiability
Percentage of NFIP claims ceded by federal government	100.00%sigi_PercentageOfUnpaidNfipClaimsCededByFederalGovernment

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