XML 49 R37.htm IDEA: XBRL DOCUMENT v3.4.0.3

Investments (Credit Concentration Risk) (Details)	Mar. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies	10.00%	10.00%

X

- Definition

Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies

+ References

No definition available.

+ Details

Name:	sigi_MaximumexposuretocreditconcentrationriskoftheCompanysstockholdersequityotherthancertainU.S.governmentagencies
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

Name:	us-gaap_InvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration