Organization	9 Months Ended
Sep. 30, 2011
Organization [Abstract]
Organization	NOTE 1. Organization Selective Insurance Group, Inc., through its subsidiaries, (collectively referred to as "we," "us," or "our") offers property and casualty insurance products. Selective Insurance Group, Inc. (referred to as the "Parent") was incorporated in New Jersey in 1977 and its main offices are located in Branchville, New Jersey. The Parent's common stock is publicly traded on the NASDAQ Global Select Market under the symbol "SIGI." We classify our business into two operating segments: Insurance Operations, which sells property and casualty insurance products and services primarily in 22 states in the Eastern and Midwestern U.S.; and Investments.

The entire disclosure for the nature of an entity's business, the major products or services it sells or provides and its principal markets, including the locations of those markets. If the entity operates in more than one business, the disclosure also indicates the relative importance of its operations in each business and the basis for the determination (for example, assets, revenues, or earnings).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Name Statement of Position (SOP)

-Publisher AICPA

-Number 94-6

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6003-108592

Name:	us-gaap_NatureOfOperations
Namespace Prefix:	us-gaap
Data Type:	nonnum:textBlockItemType
Balance Type:	na
Period Type:	duration