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Adoption of Accounting Pronouncements(Other Information) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Underwriting loss	$ (103,584)
Combined Ratio	107.20%
As Originally Reported [Member]
Underwriting loss	$ (106,919)
Combined Ratio	107.40%

X

- Definition

Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

+ References

No definition available.

+ Details

Name:	sigi_Underwritinglossprofitgross
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned.

+ References

No definition available.

+ Details

Name:	us-gaap_CombinedRatio
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration