XML 122 R60.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 Securities
Number of securities in an unrealized/unrecognized loss position | 350 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition | 557 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition
Unrealized Unrecognized Loss | 10,596,000 sigi_UnrealizedUnrecognizedLoss | $ 52,011,000 sigi_UnrealizedUnrecognizedLoss
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months | 8,200,000 sigi_SecuritiesInUnrealizedUnrecognizedLossPositionForMoreThanTwelveMonthsUnrealizedUnrecognizedLoss | 2,900,000 sigi_SecuritiesInUnrealizedUnrecognizedLossPositionForMoreThanTwelveMonthsUnrealizedUnrecognizedLoss
Number Of Securities With Unrealized Unrecognized Losses Over 12 Months That Experienced Rating Downgrade | 1 sigi_NumberOfSecuritiesWithUnrealizedUnrecognizedLossesOver12MonthsThatExperiencedRatingDowngrade |
Percentage Decline in Fair Value Percentage | 2.00%sigi_PercentageDeclineInFairValuePercentage | 6.00%sigi_PercentageDeclineInFairValuePercentage
Eighty Percent to Ninety-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 350 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember | 556 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember
Unrealized Unrecognized Loss | 10,596,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember | 51,835,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_EightyPercentToNinetyNinePercentMember
Sixty Percent to Seventy-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 0 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember | 1 sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember
Unrealized Unrecognized Loss | 0 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember | 176,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SixtyPercentToSeventyNinePercentMember
Forty Percent to Fifty-Nine Percent [Member] |  |
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] |  |
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] |  |
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] |  |
Increase of Yield on Note | 86 sigi_DebtInstrumentsInterestRateIncreaseDecrease / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = us-gaap_USTreasuryNotesSecuritiesMember |
Securities With Rating Downgrades [Member] |  |
Percentage Decline in Fair Value Percentage | 13.00%sigi_PercentageDeclineInFairValuePercentage / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SecuritiesWithRatingDowngradesMember | 9.00%sigi_PercentageDeclineInFairValuePercentage / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SecuritiesWithRatingDowngradesMember
Securities Rating Downgrade [Member] |  |
Unrealized Unrecognized Loss | 1,100,000 sigi_UnrealizedUnrecognizedLoss / sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis = sigi_SecuritiesRatingDowngradeMember |

X
- Definition Increase (decrease) in basis points of the stated interest rate on the debt instrument. + References No definition available. + Details
Name: | sigi_DebtInstrumentsInterestRateIncreaseDecrease
Namespace Prefix: | sigi_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

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
Balance Type: | na
Period Type: | instant

X
- Definition Number of securities that have been in a loss position for more than twelve months that have experienced a rating downgrade during the period. + References No definition available. + Details
Name: | sigi_NumberOfSecuritiesWithUnrealizedUnrecognizedLossesOver12MonthsThatExperiencedRatingDowngrade
Namespace Prefix: | sigi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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

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
Name: | sigi_UnrealizedUnrecognizedLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_EightyPercentToNinetyNinePercentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_FortyPercentToFiftyNinePercentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_SecuritiesRatingDowngradeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_SecuritiesWithRatingDowngradesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_SixtyPercentToSeventyNinePercentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_TwentyPercentToThirtyNinePercentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=sigi_ZeroPercentToNineteenPercentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_ScheduleOfNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionAxis=us-gaap_USTreasuryNotesSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |