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Subsequent Events Subsequent Events (Details) - Subsequent Event [Member] $ in Millions | 3 Months Ended
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Dec. 31, 2016 USD ($)
Subsequent Event [Line Items] |
Flood Claims Handling Fees | $ 1
Minimum [Member] |
Subsequent Event [Line Items] |
Estimated Catastrophe Losses | 10
Maximum [Member] |
Subsequent Event [Line Items] |
Estimated Catastrophe Losses | $ 14

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- Definition Estimated catastrophe losses as a result of severe weather conditions. + References No definition available. + Details
Name: | sigi_EstimatedCatastropheLosses
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Commissions received from the NFIP in handling flood claims. + References No definition available. + Details
Name: | sigi_FloodClaimsHandlingFees
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |