Transactions with Related Persons	12 Months Ended
Dec. 31, 2017
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie, the founder and Managing Member of Prospector Partners, LLC (“Prospector”), retired from the White Mountains 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 the Consulting Agreement described below were terminated. Pursuant to an investment management agreements with WM Advisors and OneBeacon, Prospector charged WM Advisors and OneBeacon fees based on the amount of assets under management. During 2015, Prospector earned $2.1 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. Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector. For the year ended December 31, 2015, White Mountains and OneBeacon incurred $0.4 million in management fees and $0.1 million in incentive fees. Other Relationships and Transactions During 2017, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company. On July 13, 2017, the Company repurchased 235,000 White Mountains common shares for $850.00 per share, the market price at the time the agreement was reached. During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 White Mountains 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 White Mountains 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 in two transactions. On June 1, 2015, the Company repurchased 19,688 White Mountains 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 White Mountains common shares for $761.50, the market price at the time the agreement was reached. 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, WM Advisors earned $8.0 million in fees from Symetra.

Transactions with Related Persons

Prospector

Mr. John Gillespie, the founder and Managing Member of Prospector Partners, LLC (“Prospector”), retired from the White Mountains 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 the Consulting Agreement described below were terminated.

Pursuant to an investment management agreements with WM Advisors and OneBeacon, Prospector charged WM Advisors and OneBeacon fees based on the amount of assets under management. During 2015, Prospector earned $2.1 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.

Prior to fully redeeming their interests on June 30, 2015, White Mountains and OneBeacon had interests in limited partnership investments managed by Prospector. For the year ended December 31, 2015, White Mountains and OneBeacon incurred $0.4 million in management fees and $0.1 million in incentive fees.

Other Relationships and Transactions

During 2017, the Company repurchased shares from Franklin Mutual Advisers, a beneficial owner of the Company. On July 13, 2017, the Company repurchased 235,000 White Mountains common shares for $850.00 per share, the market price at the time the agreement was reached.

During 2016, the Company repurchased shares from Franklin Mutual Advisers in two transactions. On April 19, 2016, the Company repurchased 325,000 White Mountains 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 White Mountains 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 in two transactions. On June 1, 2015, the Company repurchased 19,688 White Mountains 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 White Mountains common shares for $761.50, the market price at the time the agreement was reached.

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, WM Advisors earned $8.0 million in fees from Symetra.