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Segment Information Narrative (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Number of Operating Segments 4us-gaap_NumberOfOperatingSegments  
Gross premiums of self-insured pooled group renewal book of business sold   $ 37,000,000sigi_GrossPremiumsWrittenofSelfInsuredPooledGroupBookOfBusinessSold
Proceeds from sale of renewal rights $ 0us-gaap_ProceedsFromDivestitureOfBusinesses $ 8,000,000us-gaap_ProceedsFromDivestitureOfBusinesses