Related Party Transactions	12 Months Ended
Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions William M. Rue, a Director of the Parent, is Chairman of, and owns more than 10% of the equity of, Chas. E. Rue & Son, Inc., t/a Rue Insurance, a general independent retail insurance agency ("Rue Insurance"). Rue Insurance is an appointed distribution partner of the Insurance Subsidiaries on terms and conditions similar to those of our other distribution partners, which includes the right to participate in the Agent Plan. Mr. Rue’s son is President, and an employee, of Rue Insurance and Mr. Rue’s daughter is an employee of Rue Insurance. Our relationship with Rue Insurance has existed since 1928. Rue Insurance placed insurance policies with the Insurance Subsidiaries for its customers and itself. Direct premiums written associated with these policies were $11.1 million in 2017, $10.4 million in 2016, and $9.6 million in 2015. In return, the Insurance Subsidiaries paid standard market commissions, including supplemental commissions, to Rue Insurance of $2.3 million in 2017, $2.1 million in 2016, and $1.7 million in 2015. Amounts due to Rue Insurance at December 31, 2017 and December 31, 2016 were $0.6 million and $0.7 million, respectively. In 2005, we established a private foundation, now named The Selective Insurance Group Foundation (the "Foundation"), under Section 501(c)(3) of the Internal Revenue Code. The Board of Directors of the Foundation is comprised of some of the Parent's officers. We made less than $0.1 million of contributions and no contributions to the Foundation in 2017 and 2016, respectively. We made contributions to the Foundation in the amount of $1.0 million in 2015. BlackRock, Inc., a leading publicly traded investment management firm (“BlackRock”), has purchased our common shares in the ordinary course of its investment business and has previously filed Schedules 13G/A with the SEC. On January 19, 2018, BlackRock filed a Schedule 13G/A reporting beneficial ownership as of December 31, 2017, of 12.8% of our common stock. In connection with purchasing our common shares, BlackRock filed the necessary filings with insurance regulatory authorities. On the basis of those filings, BlackRock is deemed not to be a controlling person for the purposes of applicable insurance law. We are required to disclose related party information for our transactions with BlackRock. BlackRock is highly regulated, serves its clients as a fiduciary, and has a diverse platform of active (alpha) and index (beta) investment strategies across asset classes that enables it to tailor investment outcomes and asset allocation solutions for clients. BlackRock also offers the BlackRock Solutions^® investment and risk management technology platform, Aladdin^®, risk analytics, advisory, and technology services and solutions to a broad base of institutional and wealth management investors. In 2017 and 2016, we incurred expenses related to BlackRock of $2.0 million and $0.4 million, respectively, for services rendered. Amounts payable for such services at December 31, 2017 and December 31, 2016, were $0.5 million and $0.4 million, respectively. All contracts with BlackRock were consummated in the ordinary course of business on an arm's-length basis.

Related Party Transactions

William M. Rue, a Director of the Parent, is Chairman of, and owns more than 10% of the equity of, Chas. E. Rue & Son, Inc., t/a Rue Insurance, a general independent retail insurance agency ("Rue Insurance"). Rue Insurance is an appointed distribution partner of the Insurance Subsidiaries on terms and conditions similar to those of our other distribution partners, which includes the right to participate in the Agent Plan. Mr. Rue’s son is President, and an employee, of Rue Insurance and Mr. Rue’s daughter is an employee of Rue Insurance. Our relationship with Rue Insurance has existed since 1928.

Rue Insurance placed insurance policies with the Insurance Subsidiaries for its customers and itself. Direct premiums written associated with these policies were $11.1 million in 2017, $10.4 million in 2016, and $9.6 million in 2015. In return, the Insurance Subsidiaries paid standard market commissions, including supplemental commissions, to Rue Insurance of $2.3 million in 2017, $2.1 million in 2016, and $1.7 million in 2015. Amounts due to Rue Insurance at December 31, 2017 and December 31, 2016 were $0.6 million and $0.7 million, respectively.

In 2005, we established a private foundation, now named The Selective Insurance Group Foundation (the "Foundation"), under Section 501(c)(3) of the Internal Revenue Code. The Board of Directors of the Foundation is comprised of some of the Parent's officers. We made less than $0.1 million of contributions and no contributions to the Foundation in 2017 and 2016, respectively. We made contributions to the Foundation in the amount of $1.0 million in 2015.

BlackRock, Inc., a leading publicly traded investment management firm (“BlackRock”), has purchased our common shares in the ordinary course of its investment business and has previously filed Schedules 13G/A with the SEC. On January 19, 2018, BlackRock filed a Schedule 13G/A reporting beneficial ownership as of December 31, 2017, of 12.8% of our common stock. In connection with purchasing our common shares, BlackRock filed the necessary filings with insurance regulatory authorities. On the basis of those filings, BlackRock is deemed not to be a controlling person for the purposes of applicable insurance law.

We are required to disclose related party information for our transactions with BlackRock. BlackRock is highly regulated, serves its clients as a fiduciary, and has a diverse platform of active (alpha) and index (beta) investment strategies across asset classes that enables it to tailor investment outcomes and asset allocation solutions for clients. BlackRock also offers the BlackRock Solutions^® investment and risk management technology platform, Aladdin^®, risk analytics, advisory, and technology services and solutions to a broad base of institutional and wealth management investors. In 2017 and 2016, we incurred expenses related to BlackRock of $2.0 million and $0.4 million, respectively, for services rendered. Amounts payable for such services at December 31, 2017 and December 31, 2016, were $0.5 million and $0.4 million, respectively. All contracts with BlackRock were consummated in the ordinary course of business on an arm's-length basis.