Transactions with Related Persons	12 Months Ended
Dec. 31, 2012
Related Party Transactions [Abstract]
Transactions with Related Persons	Transactions with Related Persons Prospector Mr. John Gillespie is the founder and Managing Member of Prospector. Prospector serves as a discretionary adviser with respect to specified assets, primarily equity securities, managed by WM Advisors on behalf of White Mountains and other clients of WM Advisors. Pursuant to an investment management agreement with WM Advisors (the “WMA Agreement”), Prospector charges WM Advisors fees based on the following schedule: 100 basis points on the first $200.0 million of assets under management; 50 basis points on the next $200.0 million; and 25 basis points on amounts over $400.0 million. At December 31, 2012, Prospector managed a total of $465.9 million of assets for White Mountains (excluding OneBeacon and Symetra) under this arrangement. Prospector has a separate investment management agreement with Symetra that began on July 1, 2010. Until that date, Symetra was a party to the WMA Agreement and subject to the above fee schedule. Prospector has separate investment management agreements with OneBeacon (the “OneBeacon Agreements”) pursuant to which Prospector supervises and directs specified assets, primarily equity securities, including assets in OneBeacon’s defined benefit and defined contribution plans (the “ERISA Assets”). All assets managed under the OneBeacon Agreements are subject to a single fee schedule that is substantially similar to the terms of the WMA Agreement fee schedule. At December 31, 2012, Prospector managed $533.6 million of assets for OneBeacon under this arrangement, including $205.9 million of ERISA Assets. During 2012, 2011 and 2010, Prospector earned $6.4 million, $6.0 million, and $6.6 million in total fees pursuant to the WMA Agreement and the OneBeacon Agreements. Prospector also advises 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 7,000 performance shares for the 2013-2015 cycle, 7,750 performance shares for the 2012-2014 cycle and 8,500 performance shares for the 2011-2013 performance cycle. In accordance with the terms of the WTM Incentive Plan, performance against target governing the performance shares will be confirmed by the Compensation Committee of the Board following the end of each performance cycle and the number of performance shares actually awarded at that time will range from 0% to 200% of the target number granted. Based on the Company’s performance, Prospector received 49% of the 9,200 performance shares granted for the 2010-2012 performance cycle for a total payout of $2.5 million. Unless and until the Consulting Agreement has been terminated, and subject to the approval of the Compensation Committee, at the beginning of each performance cycle Prospector is to be granted performance shares with a value of approximately $4.5 million. The Compensation Committee establishes the performance target for such performance shares. Pursuant to a revenue sharing agreement, Prospector has agreed to pay White Mountains 6% of the revenues in excess of $500,000 of certain of Prospector’s funds in return for White Mountains having made a founding investment in 1997. During 2012, 2011 and 2010, White Mountains earned $0.5 million, $0.2 million, and $0.7 million under this arrangement. At December 31, 2012, White Mountains had $117.2 million invested in limited partnership investment interests managed by Prospector (the “Funds”). This total includes $12.9 million of OneBeacon assets. Under the limited partnership agreements, Prospector serves as general partner and general manager of the Funds and is paid management and incentive performance fees by White Mountains and OneBeacon. For the year ended December 31, 2012, White Mountains and OneBeacon incurred $1.1 million in management fees and $1.3 million in incentive fees. In addition, Messrs. Barrette, Davis, Gillespie and Waters, each a director of the Company, and Mr. Campbell, an executive officer of the Company, owned limited partnership investment interests managed by Prospector as of such date. Oakum Bay Capital Mr. Kernan Oberting, a Managing Director of White Mountains Capital, is the Founder, Chairman of the Board and Chairman of the Risk Committee of Oakum Bay Capital (“OBC”). OBC serves as the general manager of, and owns general partnership interests in, the KVO Capital Partners (“KCP”) and the Trimarc Capital Fund (“TCF”), which are hedge funds with an aggregate of $66 million under management as of December 31, 2012. Mr. Brooke Parish, who is Mr. Oberting's brother-in-law, is the CEO of OBC. Mr. David Oberting, who is Mr. Oberting's brother, is the portfolio manager of KCP. In connection with Mr. Oberting commencing employment with White Mountains Capital, on July 16, 2012, the Company purchased $2 million of preferred equity and received 7.5% of the common equity of OBC. The preferred equity is entitled to dividends at a rate of 5% per annum, increasing to 10% per annum after four years, and is mandatorily redeemable after six years. After giving effect to the common equity investment, Mr. Oberting and Mr. Parish beneficially owned 27.5% and 45%, respectively, of the common equity of OBC. In addition, at the time it purchased the preferred equity in OBC, the Company invested $6 million in TCF. In 2010, the Company had invested $4 million in TCF. As of December 31, 2012, White Mountains had $9.9 million invested in TCF. Under TCF's limited partnership agreements, OBC is paid management and incentive performance fees by White Mountains. For the year ended December 31, 2012, White Mountains paid $56,811 of management fees and $88,090 of incentive fees. OBC receives the management fees and 40% of the incentive fees. Mr. Oberting has limited partnership investments in KCP and TCF. Mr. Barrette had a limited partnership investment in KCP that was redeemed in full in September 2012. Other Relationships and Transactions WMA provides investment advisory and management services to Symetra. At December 31, 2012 and 2011, WMA had $27.4 billion and $26.3 billion of assets under management from Symetra. During 2012, 2011 and 2010, WMA earned $15.9 million, $15.2 million and $13.8 million in fees from Symetra. On July 12, 2012, the Company repurchased 50,000 WTM common shares from John J. Byrne, a beneficial owner of the Company. The price per share was $528.45, the market price at the time the agreement was reached. On December 19, 2012, the Company repurchased 285,000 WTM common shares from mutual funds managed by Franklin Mutual Advisers, a beneficial owner of the Company. The price per share was $519.06, the market price at the time the agreement was reached. On January 7, 2011, the Company repurchased 140,000 WTM common shares from mutual funds managed by Franklin Mutual Advisers at a price per share of $342.50, which was $2.50 less than the market price at the time the agreement was reached. Mr. Gillespie, a director of the Company, indirectly through general and limited partnership interests holds a 33% interest in Dowling & Partners Connecticut Fund III, LP (“Fund III”). Two of the Company’s indirect subsidiaries, OneBeacon Professional Insurance (“OB Professional Insurance”) and White Mountains Specialty Underwriting, Inc. (“WMSUI”), had previously borrowed approximately $8.0 million and $7.0 million, respectively, from Fund III in connection with an incentive program sponsored by the State of Connecticut known as the Connecticut Insurance Reinvestment Act (the “CIR Act”). The CIR Act provides for Connecticut income tax credits to be granted for qualifying investments made by approved fund managers. Both loans were repaid in full during 2006. The loans were qualifying investments which generated tax credits to be shared equally between Fund III on the one hand and OB Professional Insurance and WMSUI on the other. As a result of his interest in Fund III, during 2009, Mr. Gillespie generated approximately $0.5 million in such tax credits.

Transactions with Related Persons

Prospector

Mr. John Gillespie is the founder and Managing Member of Prospector. Prospector serves as a discretionary adviser with respect to specified assets, primarily equity securities, managed by WM Advisors on behalf of White Mountains and other clients of WM Advisors.

Pursuant to an investment management agreement with WM Advisors (the “WMA Agreement”), Prospector charges WM Advisors fees based on the following schedule: 100 basis points on the first $200.0 million of assets under management; 50 basis points on the next $200.0 million; and 25 basis points on amounts over $400.0 million. At December 31, 2012, Prospector managed a total of $465.9 million of assets for White Mountains (excluding OneBeacon and Symetra) under this arrangement. Prospector has a separate investment management agreement with Symetra that began on July 1, 2010. Until that date, Symetra was a party to the WMA Agreement and subject to the above fee schedule.

Prospector has separate investment management agreements with OneBeacon (the “OneBeacon Agreements”) pursuant to which Prospector supervises and directs specified assets, primarily equity securities, including assets in OneBeacon’s defined benefit and defined contribution plans (the “ERISA Assets”). All assets managed under the OneBeacon Agreements are subject to a single fee schedule that is substantially similar to the terms of the WMA Agreement fee schedule. At December 31, 2012, Prospector managed $533.6 million of assets for OneBeacon under this arrangement, including $205.9 million of ERISA Assets.

During 2012, 2011 and 2010, Prospector earned $6.4 million, $6.0 million, and $6.6 million in total fees pursuant to the WMA Agreement and the OneBeacon Agreements.

Prospector also advises 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 7,000 performance shares for the 2013-2015 cycle, 7,750 performance shares for the 2012-2014 cycle and 8,500 performance shares for the 2011-2013 performance cycle. In accordance with the terms of the WTM Incentive Plan, performance against target governing the performance shares will be confirmed by the Compensation Committee of the Board following the end of each performance cycle and the number of performance shares actually awarded at that time will range from 0% to 200% of the target number granted. Based on the Company’s performance, Prospector received 49% of the 9,200 performance shares granted for the 2010-2012 performance cycle for a total payout of $2.5 million. Unless and until the Consulting Agreement has been terminated, and subject to the approval of the Compensation Committee, at the beginning of each performance cycle Prospector is to be granted performance shares with a value of approximately $4.5 million. The Compensation Committee establishes the performance target for such performance shares.

Pursuant to a revenue sharing agreement, Prospector has agreed to pay White Mountains 6% of the revenues in excess of $500,000 of certain of Prospector’s funds in return for White Mountains having made a founding investment in 1997. During 2012, 2011 and 2010, White Mountains earned $0.5 million, $0.2 million, and $0.7 million under this arrangement.

At December 31, 2012, White Mountains had $117.2 million invested in limited partnership investment interests managed by Prospector (the “Funds”). This total includes $12.9 million of OneBeacon assets. Under the limited partnership agreements, Prospector serves as general partner and general manager of the Funds and is paid management and incentive performance fees by White Mountains and OneBeacon. For the year ended December 31, 2012, White Mountains and OneBeacon incurred $1.1 million in management fees and $1.3 million in incentive fees. In addition, Messrs. Barrette, Davis, Gillespie and Waters, each a director of the Company, and Mr. Campbell, an executive officer of the Company, owned limited partnership investment interests managed by Prospector as of such date.

Oakum Bay Capital

Mr. Kernan Oberting, a Managing Director of White Mountains Capital, is the Founder, Chairman of the Board and Chairman of the Risk Committee of Oakum Bay Capital (“OBC”). OBC serves as the general manager of, and owns general partnership interests in, the KVO Capital Partners (“KCP”) and the Trimarc Capital Fund (“TCF”), which are hedge funds with an aggregate of $66 million under management as of December 31, 2012. Mr. Brooke Parish, who is Mr. Oberting's brother-in-law, is the CEO of OBC. Mr. David Oberting, who is Mr. Oberting's brother, is the portfolio manager of KCP.

In connection with Mr. Oberting commencing employment with White Mountains Capital, on July 16, 2012, the Company purchased $2 million of preferred equity and received 7.5% of the common equity of OBC. The preferred equity is entitled to dividends at a rate of 5% per annum, increasing to 10% per annum after four years, and is mandatorily redeemable after six years. After giving effect to the common equity investment, Mr. Oberting and Mr. Parish beneficially owned 27.5% and 45%, respectively, of the common equity of OBC.

In addition, at the time it purchased the preferred equity in OBC, the Company invested $6 million in TCF. In 2010, the Company had invested $4 million in TCF. As of December 31, 2012, White Mountains had $9.9 million invested in TCF. Under TCF's limited partnership agreements, OBC is paid management and incentive performance fees by White Mountains.

For the year ended December 31, 2012, White Mountains paid $56,811 of management fees and $88,090 of incentive fees. OBC receives the management fees and 40% of the incentive fees. Mr. Oberting has limited partnership investments in KCP and TCF. Mr. Barrette had a limited partnership investment in KCP that was redeemed in full in September 2012.

Other Relationships and Transactions

WMA provides investment advisory and management services to Symetra. At December 31, 2012 and 2011, WMA had $27.4 billion and $26.3 billion of assets under management from Symetra. During 2012, 2011 and 2010, WMA earned $15.9 million, $15.2 million and $13.8 million in fees from Symetra.

On July 12, 2012, the Company repurchased 50,000 WTM common shares from John J. Byrne, a beneficial owner of the Company. The price per share was $528.45, the market price at the time the agreement was reached.

On December 19, 2012, the Company repurchased 285,000 WTM common shares from mutual funds managed by Franklin Mutual Advisers, a beneficial owner of the Company. The price per share was $519.06, the market price at the time the agreement was reached.

On January 7, 2011, the Company repurchased 140,000 WTM common shares from mutual funds managed by Franklin Mutual Advisers at a price per share of $342.50, which was $2.50 less than the market price at the time the agreement was reached.

Mr. Gillespie, a director of the Company, indirectly through general and limited partnership interests holds a 33% interest in Dowling & Partners Connecticut Fund III, LP (“Fund III”). Two of the Company’s indirect subsidiaries, OneBeacon Professional Insurance (“OB Professional Insurance”) and White Mountains Specialty Underwriting, Inc. (“WMSUI”), had previously borrowed approximately $8.0 million and $7.0 million, respectively, from Fund III in connection with an incentive program sponsored by the State of Connecticut known as the Connecticut Insurance Reinvestment Act (the “CIR Act”). The CIR Act provides for Connecticut income tax credits to be granted for qualifying investments made by approved fund managers. Both loans were repaid in full during 2006. The loans were qualifying investments which generated tax credits to be shared equally between Fund III on the one hand and OB Professional Insurance and WMSUI on the other. As a result of his interest in Fund III, during 2009, Mr. Gillespie generated approximately $0.5 million in such tax credits.