Related Party Transactions (Narrative) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity ownership percentage of director of parent of general independent retail insurance agency	10.00%
Direct premiums written	$ 2,228,270,000us-gaap_DirectPremiumsWritten	$ 2,133,793,000us-gaap_DirectPremiumsWritten	$ 1,955,667,000us-gaap_DirectPremiumsWritten
Premiums paid to Rue Insurance for insurance coverage	0	0	200,000
Contributions to the Selective Group Foundation	800,000	400,000	400,000
Related Party Insurance Agency [Member]
Direct premiums written	9,000,000	8,200,000	7,700,000
Commissions paid to Rue Insurance	$ 1,600,000	$ 1,300,000	$ 1,300,000

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

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The amount of premiums paid on insurance placed by a related party.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- 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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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),(c)(1)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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Balance Type:	credit
Period Type:	duration

- 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=31042434&loc=d3e3536-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Period Type: