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Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity ownership percentage of director of parent of general independent retail insurance agency	10.00%
Direct premiums written	$ 2,403,519	$ 2,228,270	$ 2,133,793
Contributions to the Selective Insurance Group Foundation	1,000	800	400
Related Party Insurance Agency [Member]
Direct premiums written	9,600	9,000	8,200
Commissions paid to Rue Insurance	$ 1,700	$ 1,600	$ 1,300

X

- Definition

The minimum percentage of the equity of an independent insurance agency owned by a director of the Parent.

+ References

No definition available.

+ Details

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

X

- Definition

The amount of contributions made to a related party private foundation.

+ References

No definition available.

+ Details

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

X

- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b),(c)(1)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 17
-Article 12

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of cash paid for commissions during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3536-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: