Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2019
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 owns more than 10% of the equity of Rue Insurance. 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 $11.0 million in 2019, $10.1 million in 2018, and $11.1 million in 2017. In return, the Insurance Subsidiaries paid standard market commissions, including supplemental commissions, to Rue Insurance of $2.0 million in 2019, $2.1 million in 2018, and $2.3 million in 2017. Amounts due to Rue Insurance at December 31, 2019 and December 31, 2018 were $0.3 million and $0.4 million, respectively. All contracts and transactions with Rue Insurance were consummated in the ordinary course of business on an arm's-length basis. 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 $1.3 million of contributions to the Foundation in 2019, $0.5 million in 2018, and no contributions in 2017. 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 February 4, 2020, BlackRock filed a Schedule 13G/A reporting beneficial ownership as of December 31, 2019, of 11.7% 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. We incurred expenses related to BlackRock for services rendered of $2.2 million in 2019, and $2.0 million in both 2018 and 2017. Amounts payable for such services at December 31, 2019 and December 31, 2018, were $1.1 million and $1.0 million, respectively. As part of our overall investment diversification, we invest in various BlackRock funds from time to time. These funds accounted for less than 1% of our invested assets at December 31, 2019 and December 31, 2018, and are predominately reflected in Equity securities on our Consolidated Balance Sheet. During 2019, with regard to BlackRock funds, we (i) purchased $21.7 million, (ii) sold $59.5 million, (iii) recognized a $5.7 million net realized and unrealized gain, and (iv) recorded $0.8 million in income. During 2018, we purchased $41.4 million in securities and recognized net realized and unrealized losses of $3.6 million. There were no material transactions related to these holdings in 2017. There were no amounts payable on the settlement of these investment transactions at December 31, 2019, December 31, 2018, or December 31, 2017. Our Pension Plan's investment portfolio contained investments in BlackRock funds of $144.2 million at December 31, 2019 and $131.9 million at December 31, 2018. During 2019, with regard to BlackRock funds, the Pension Plan (i) purchased $19.7 million, (ii) sold $44.1 million, and (iii) recorded net investment income of $36.7 million. In 2018, with regard to BlackRock funds, the Pension Plan (i) purchased $132.5 million, (ii) sold $125.6 million, and (iii) recorded net investment income of $9.3 million. In 2017, with regard to BlackRock funds, the Pension Plan (i) purchased $10.0 million, (ii) sold $4.1 million, and (iii) recorded net investment income of $25.2 million. In addition, our Deferred Compensation Plan and Retirement Savings Plan may offer our employees the option to invest in various BlackRock funds. All contracts and transactions with BlackRock were consummated in the ordinary course of business on an arm's-length basis. As of December 31, 2019, the Vanguard Group ("Vanguard") held 9.4% of our common stock. Vanguard is one of the world's largest investment management companies, offering low cost mutual funds, exchange-trade funds ("ETFs"), and other investment related services. On January 10, 2019, Vanguard filed a Schedule 13G/A reporting beneficial ownership as of December 31, 2018, of 10.1% of our common stock. In connection with purchasing our common shares in the prior year, Vanguard filed the necessary filings with insurance regulatory authorities to disclaim control and we do not expect Vanguard to be deemed a controlling person by any insurance regulator. As part of our overall investment diversification, we may invest in various Vanguard funds from time to time. As of December 31, 2019, we had no investments in Vanguard funds, and our investment in these funds at December 31, 2018 was less than 1% of invested assets at that time and was recorded within Equity securities on our Consolidated Balance Sheet. During 2019, we sold $11.8 million of a Vanguard ETF, recorded dividend income of $0.2 million and recorded capital gains of $1.3 million from such ETF, with no amounts receivable on the settlement of this transaction at December 31, 2019. During 2018, we purchased $11.5 million of a Vanguard ETF and recorded dividend income of $0.4 million from such ETF, with no amounts payable on the settlement of this transaction at December 31, 2018. We had no transactions with Vanguard in 2017. Our Pension Plan's investment portfolio contained no investments in Vanguard funds at December 31, 2019 and December 31, 2018. Our Pension Plan's investment portfolio contained investments in Vanguard funds of $86.3 million in 2017. The Pension Plan had no transactions with Vanguard in 2019. During 2018 and 2017, the Pension Plan purchased $8.4 million and $5.2 million of Vanguard funds, respectively. In 2018, the Pension Plan sold $85.4 million of Vanguard funds. The Pension Plan recorded a net investment loss on Vanguard funds of $5.5 million in 2018, and net investment income of $9.1 million in 2017. In addition, our Deferred Compensation Plan and Retirement Savings Plan may offer our employees the option to invest in various Vanguard funds. All transactions with Vanguard are consummated in the ordinary course of business on an arm's-length basis.