Investments (Credit Concentration Risk) (Details)	Mar. 31, 2021	Dec. 31, 2020
Investments [Abstract]
Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government-backed investments	10.00%	10.00%

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

No definition available.

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Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant