Transactions with Related Persons	12 Months Ended
Transactions with Related Persons	Dec. 31, 2016
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie, the founder and Managing Member of Prospector, retired from the WTM Board of Directors in May 2015. Until June 30, 2015, Prospector served as a discretionary adviser with respect to specified assets, primarily common equity securities and convertible fixed maturity investments, managed directly or through WM Advisors on behalf of White Mountains and other clients of WM Advisors. At that time, the investment management agreements between WM Advisors and Prospector and OneBeacon and Prospector and the Consulting Agreement described below were terminated. Pursuant to an investment management agreements with WM Advisors (the “WMA Agreement”) and OneBeacon (the “OneBeacon Agreement”), Prospector charged WM Advisors and OneBeacon fees based on the following schedule: 100 basis points on the first $200 million of assets under management; 50 basis points on the next $200 million; and 25 basis points on amounts over $400 million. During 2015 and 2014, Prospector earned $2.1 million and $6.5 million in total fees pursuant to the investment management agreements with WM Advisors and OneBeacon. Prospector also had a separate investment management agreement with Symetra that was terminated in the fourth quarter of 2015. Prospector also advised White Mountains on matters including capital management, asset allocation, private equity investments and mergers and acquisitions. Pursuant to a Consulting Agreement for those services, Prospector was granted 6,250 performance shares for the 2014-2016 cycle and 7,000 performance shares for the 2013-2015 cycle. Under the terms of the Consulting Agreement, Prospector earned a prorated portion of the outstanding performance share grants at the time of the termination of the Consulting Agreement and was paid $5.5 million in respect thereof. Pursuant to a pre-existing revenue sharing agreement (the “Revenue Agreement”), Prospector paid White Mountains 6% of the annual revenues in excess of $500,000 of certain of Prospector’s funds in return for White Mountains having made a founding investment in 1997. White Mountains and Prospector mutually agreed to terminate the Revenue Agreement as of June 30, 2015. During 2014, White Mountains earned $0.4 million under the Revenue Agreement. No amount was earned under the Revenue Agreement in 2015. Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector (the “Funds”). For the years ended December 31, 2015 and 2014, White Mountains and OneBeacon incurred $0.4 million and $1.1 million in management fees and $0.1 million and $0.7 million in incentive fees. Other Relationships and Transactions WM Advisors provided investment advisory and management services to Symetra through the fourth quarter of 2016. In August 2015, Symetra announced it had entered into a merger agreement with Sumitomo Life pursuant to which Sumitomo Life would acquire all of the outstanding shares of Symetra. Following the announcement and Symetra shareholders’ November 5, 2015 meeting to approve the transaction, White Mountains relinquished its representation on Symetra’s board of directors and Symetra was no longer a related party to White Mountains. During 2015 and 2014, WMA earned $8.0 million and $18.4 million in fees from Symetra. During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 WTM common shares for $807.00 per share, the market price at the time the agreement was reached. On September 15, 2016, the Company repurchased 305,000 WTM common shares for $820.00 per share, the market price at the time the agreement was reached. During 2015, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company, in two transactions. On June 1, 2015, the Company repurchased 19,688 WTM common shares for $650.03 per share, the market price at the time the agreement was reached. On September 17, 2015, the Company repurchased 26,300 WTM common shares for $761.50, the market price at the time the agreement was reached.