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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2013 Securities | Dec. 31, 2012 Securities
Schedule of Available-for-sale Securities [Line Items] |  |
Unrealized Unrecognized Losses Period Increase | $ (40,600,000) |
Number of securities in an unrealized/unrecognized loss position | 502 | 101
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months | 1,100,000 | 1,700,000
Unrealized Unrecognized Loss | 43,581,000 | 2,934,000
Eighty Percent to Ninety-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number of securities in an unrealized/unrecognized loss position | 501 | 100
Unrealized Unrecognized Loss | 43,295,000 | 2,701,000
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 | 1
Unrealized Unrecognized Loss | 0 | 233,000
Forty Percent to Fifty-Nine Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number of securities in an unrealized/unrecognized loss position | 1 | 0
Unrealized Unrecognized Loss | 286,000 | 0
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 | 0
Unrealized Unrecognized Loss | 0 | 0
Zero Percent to Nineteen Percent [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number of securities in an unrealized/unrecognized loss position | 0 | 0
Unrealized Unrecognized Loss | $ 0 | $ 0
10 year U.S. Treasury Note [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Debt Instruments, Interest Rate, Increase (Decrease) | 85 |

X
- Definition Increase (decrease) in basis points of the stated interest rate on the debt instrument. + References No definition available. + Details
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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_
Data Type: | xbrli:integerItemType
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X
- 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

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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- 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
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- Details
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Balance Type: | na
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