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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) | 6 Months Ended |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended
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Jun. 30, 2013 Securities | Dec. 31, 2012 Securities | Jun. 30, 2013 Eighty Percent to Ninety-Nine Percent [Member] Securities | Dec. 31, 2012 Eighty Percent to Ninety-Nine Percent [Member] Securities | Jun. 30, 2013 Sixty Percent to Seventy-Nine Percent [Member] Securities | Dec. 31, 2012 Sixty Percent to Seventy-Nine Percent [Member] Securities | Jun. 30, 2013 Forty Percent to Fifty-Nine Percent [Member] Securities | Dec. 31, 2012 Forty Percent to Fifty-Nine Percent [Member] Securities | Jun. 30, 2013 Twenty Percent to Thirty-Nine Percent [Member] Securities | Dec. 31, 2012 Twenty Percent to Thirty-Nine Percent [Member] Securities | Jun. 30, 2013 Zero Percent to Nineteen Percent [Member] Securities | Dec. 31, 2012 Zero Percent to Nineteen Percent [Member] Securities | Jun. 30, 2013 10 year U.S. Treasury Note [Member]
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized Unrecognized Losses Period Increase | $ (43,300,000) |  |  |  |  |  |  |  |  |  |  |  |
Number of securities in an unrealized/unrecognized loss position | 505 | 101 | 504 | 100 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months | 1,000,000 | 1,700,000 |  |  |  |  |  |  |  |  |  |  |
Increase in interest rate on the 10 year US treasury note |  |  |  |  |  |  |  |  |  |  |  |  | 64
Unrealized Unrecognized Loss | 46,260,000 | 2,934,000 | 46,022,000 | 2,701,000 | 238,000 | 233,000 | 0 | 0 | 0 | 0 | 0 | 0 |
Non-Credit OTTI Charges |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |
Decline in fair value percentage |  | 5.00% |  |  |  |  |  |  |  |  |  |  |

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- Definition Non-credit portion of other-than-temporary impairment losses on fixed maturity securities recognized in other comprehensive income that have been in a loss position for more than twelve months. This item includes subsequent changes to these securities such as amortization on HTM securities or subsequent sales. This item does not include subsequent changes in appreciation or loss in value. + References No definition available. + Details
Name: | sigi_NonCreditPortionOfOtherThanTemporaryImpairmentLossesRecognizedInOtherComprehensiveIncomeLossPositionMoreThanTwelveMonths
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This item contains disclosure of the number of available-for-sale and held-to-maturity securities determined to be in an unrealized/unrecognized loss position. + References No definition available. + Details
Name: | sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition
Namespace Prefix: | sigi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition This item represents the percentage decline in fair value of fixed maturity securities compared to amortized cost that have been in a loss position for more than twelve months. This impairment has been determined to be temporary. + References No definition available. + Details
Name: | sigi_PercentageDeclineInFairValuePercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents total unrealized and unrecognized losses of all available-for-sale and held-to-maturity securities in an unrealized and unrecognized loss position for more than 12 months as of the end of the period. + References No definition available. + Details
Name: | sigi_SecuritiesInUnrealizedUnrecognizedLossPositionForMoreThanTwelveMonthsUnrealizedUnrecognizedLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security for securities that are in an unrecognized loss position. + References No definition available. + Details
Name: | sigi_UnrealizedUnrecognizedLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the increase or decrease in unrealized/unrecognized losses for available-for-sale and held-to-maturities securities + References No definition available. + Details
Name: | sigi_UnrealizedUnrecognizedLossesPeriodIncrease
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Increase in basis points of the stated interest rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentInterestRateIncrease
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration