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Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity ownership percentage of director of parent of general independent retail insurance agency	10.00%
Direct premiums written	$ 2,577,259	$ 2,403,519	$ 2,228,270
Due to Related Parties, Current	700	600
Contributions to the Selective Insurance Group Foundation	0	1,000	800
Related Party Insurance Agency [Member]
Direct premiums written	10,400	9,600	9,000
Commissions paid to Rue Insurance	$ 2,100	$ 1,700	$ 1,600

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

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
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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 (c)(1)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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X

- Definition

Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	credit
Period Type:	instant

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=66902209&loc=d3e3536-108585

+ Details

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Balance Type:	credit
Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: