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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) |  |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended
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Sep. 30, 2014 Securities | Dec. 31, 2013 Securities | Sep. 30, 2014 Eighty Percent to Ninety-Nine Percent [Member] Securities | Dec. 31, 2013 Eighty Percent to Ninety-Nine Percent [Member] Securities | Sep. 30, 2014 Sixty Percent to Seventy-Nine Percent [Member] Securities | Dec. 31, 2013 Sixty Percent to Seventy-Nine Percent [Member] Securities | Sep. 30, 2014 Forty Percent to Fifty-Nine Percent [Member] Securities | Dec. 31, 2013 Forty Percent to Fifty-Nine Percent [Member] Securities | Sep. 30, 2014 Twenty Percent to Thirty-Nine Percent [Member] Securities | Dec. 31, 2013 Twenty Percent to Thirty-Nine Percent [Member] Securities | Sep. 30, 2014 Zero Percent to Nineteen Percent [Member] Securities | Dec. 31, 2013 Zero Percent to Nineteen Percent [Member] Securities | Sep. 30, 2014 10 year U.S. Treasury Note [Member]
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number Of Securities In An Unrealized/Unrecognized Loss Position | 406 | 557 | 404 | 556 | 2 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
Unrealized Unrecognized Loss | $ 19,257,000 | $ 52,011,000 | $ 16,417,000 | $ 51,835,000 | $ 2,840,000 | $ 176,000 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 |
Decrease of Yield on 10-year U.S. Treasury Note |  |  |  |  |  |  |  |  |  |  |  |  | 54
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months Unrealized Unrecognized Loss | $ 13,200,000 | $ 2,900,000 |  |  |  |  |  |  |  |  |  |  |
Percentage Decline in Fair Value Percentage | 3.00% | 6.00% |  |  |  |  |  |  |  |  |  |  |

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- Definition Increase (decrease) in basis points of the stated interest rate on the debt instrument. + References No definition available. + Details
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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_
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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
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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
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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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
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Balance Type: | debit
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- Details
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