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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized Unrecognized Loss | 6,776,000 sigi_UnrealizedUnrecognizedLoss | $ 10,596,000 sigi_UnrealizedUnrecognizedLoss
Eighty Percent to Ninety-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 189 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember | 350 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember
Unrealized Unrecognized Loss | 6,776,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember | 10,596,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember
Sixty Percent to Seventy-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember
Unrealized Unrecognized Loss | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember
Forty Percent to Fifty-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_FortyPercentToFiftyNinePercentMember | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_FortyPercentToFiftyNinePercentMember
Unrealized Unrecognized Loss | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_FortyPercentToFiftyNinePercentMember | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_FortyPercentToFiftyNinePercentMember
Twenty Percent to Thirty-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_TwentyPercentToThirtyNinePercentMember | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_TwentyPercentToThirtyNinePercentMember
Unrealized Unrecognized Loss | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_TwentyPercentToThirtyNinePercentMember | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_TwentyPercentToThirtyNinePercentMember
Zero Percent to Nineteen Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_ZeroPercentToNineteenPercentMember | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_ZeroPercentToNineteenPercentMember
Unrealized Unrecognized Loss | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_ZeroPercentToNineteenPercentMember | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_ZeroPercentToNineteenPercentMember
10 year U.S. Treasury Note [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Decrease of Yield on 10-year U.S. Treasury Note | 25 sigi_DebtInstrumentsInterestRateIncreaseDecrease / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = us-gaap_USTreasuryNotesSecuritiesMember |
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 28 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = us-gaap_EquitySecuritiesMember |
Unrealized Loss on Securities | 3,300,000 us-gaap_UnrealizedLossOnSecurities / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = us-gaap_EquitySecuritiesMember |
Percentage Decline In Fair Value Cost | 3.00%sigi_PercentageDeclineInFairValueCost / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = us-gaap_EquitySecuritiesMember |

X
- Definition Increase (decrease) in basis points of the stated interest rate on the debt instrument. + References No definition available. + Details
Name: | sigi_DebtInstrumentsInterestRateIncreaseDecrease
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Data Type: | xbrli:pureItemType
Balance Type: | na
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X
- 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_
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Balance Type: | na
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X
- Definition This item represents the percentage decline in fair value of equity securities compared to cost that have been in a loss position for less than twelve months. This impairment has been determined to be temporary. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- 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
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- Details
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- Definition The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period. + References No definition available. + Details
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- Details
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