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Subsequent events Subsequent event (Narrative) (Details) (USD $) In Millions, unless otherwise specified |  | 1 Months Ended
--- | --- | ---
Dec. 31, 2013 | Jan. 31, 2014 Maximum [Member] | Jan. 31, 2014 Minimum [Member]
Estimated catastrophe losses |  | $ 32 | $ 28
State Footprint | 22 |  |

X
- 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

X
- Definition Represents the number of states in our footprint. + References No definition available. + Details
Name: | sigi_StateFootprint
Namespace Prefix: | sigi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant