XML 75 R34.htm IDEA: XBRL DOCUMENT
v2.3.0.15

Adoption of Accounting Pronouncements (Narrative) (Details) (USD $) In Millions, except Per Share data | Sep. 30, 2011
--- | ---
Adoption of Accounting Pronouncements [Abstract] |
Potential after-tax impact on stockholders' equity | $ 55
Potential after-tax impact on book value per share | $ 1

X
- Definition Potential after-tax impact on the entity's stockholders' book value per share as a result of the adoption of the accounting pronouncement. + Details
Name: | sigi_PotentialAfterTaxImpactOnEntitysBookValuePerShare
Namespace Prefix: | sigi
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Potential after-tax impact on the entity's stockholders' equity as a result of the adoption of the accounting pronouncement. + Details
Name: | sigi_PotentialAfterTaxImpactOnEntitysStockholdersEquity
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesAbstract
Namespace Prefix: | us-gaap
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration