Variable Interest Entities	12 Months Ended
Dec. 31, 2012
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities BAM BAM is a newly formed mutual insurance company. As a mutual company BAM is owned by its members and a portion of each member’s charges represents a contribution to member’s surplus. During 2012, White Mountains capitalized HG Global to fund BAM through the purchase of $503.0 million of BAM Surplus Notes. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional subordinated financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a variable interest entity (“VIE”). The proceeds from the issuance of the BAM Surplus Notes represent substantially all of the equity at risk in BAM at December 31, 2012. BAM and HG Global, through its wholly-owned subsidiary, HG Re, entered into a first loss reinsurance treaty (“FLRT”), under which HG Re will provide first loss protection up to 15% of par outstanding on each bond insured by BAM in exchange for 60% of the premium, net of a ceding commission, charged by BAM. HG Re's obligations under the FLRT are satisfied by the assets in two collateral trusts: a Regulation 114 Trust and a Supplemental Trust. Losses required to be reimbursed to BAM by HG Re are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, HG Global has the right to designate two directors for election to BAM’s board of directors. White Mountains is required to consolidate the results of BAM. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. Reciprocals Reciprocals are policyholder-owned insurance carriers organized as unincorporated associations. Each policyholder insured by the reciprocal shares risk with the other policyholders. Policyholders share profits and losses in the same proportion as the amount of insurance purchased but are not subject to assessment for net losses of the reciprocal. OneBeacon had capitalized three reciprocals by loaning funds to them in exchange for surplus notes. In 2002, OneBeacon formed New Jersey Skylands Management LLC (“NJSM”) to provide management services for a fee to New Jersey Skylands Insurance Association, a reciprocal, and its wholly-owned subsidiary New Jersey Skylands Insurance Company (together, “New Jersey Skylands Insurance”). In 2004, OneBeacon formed Houston General Management Company to provide management services for a fee to another reciprocal, Houston General Insurance. In 2006, Adirondack AIF, LLC (“AAIF”), a wholly-owned subsidiary of OneBeacon, entered into an agreement to provide management services for a fee to Adirondack Insurance, a reciprocal. OneBeacon has no ownership interest in the reciprocals. Under the provisions of ASC 810, OneBeacon had determined that each of the reciprocals qualifies as a VIE. Further, OneBeacon had determined that it is the primary beneficiary as it has both the power to direct the activities of the VIE that most significantly impact the entity’s economic performance and the obligation to absorb losses or receive benefits of the entity that could potentially be significant to the VIE as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates Houston General Insurance and, until the completion of the Personal Lines Transaction on July 1, 2010, consolidated New Jersey Skylands Insurance and Adirondack Insurance. As described in Note 2, the Personal Lines Transaction, which was completed on July 1, 2010, included the sale of NJSM and AAIF and the transfer of the surplus notes issued by New Jersey Skylands Insurance and Adirondack Insurance. Completion of the Personal Lines Transaction triggered deconsolidation of New Jersey Skylands Insurance and Adirondack Insurance. Subsequent to the Personal Lines Transaction, Houston General Insurance is OneBeacon’s only reciprocal. At December 31, 2012 and 2011, consolidated amounts related to Houston General Insurance included total assets of $97.7 million and $105.9 million, respectively, and total liabilities of $120.4 million and $126.2 million, respectively. At December 31, 2012, the net amount of capital at risk is equal to the surplus note of $23.7 million less the accumulated losses of $22.7 million which includes accrued interest on the surplus note of $19.7 million which eliminates in consolidation. Prospector Offshore Fund White Mountains has determined that the Prospector Offshore Fund, Ltd. (“the Prospector Fund”) is a VIE for which White Mountains is the primary beneficiary and is required to consolidate the Prospector Fund. At December 31, 2012 and 2011, White Mountains consolidated total assets of $151.0 million and $135.8 million and total liabilities of $51.1 million and $31.6 million of the Prospector Offshore Fund. In addition, at December 31, 2012 and 2011, White Mountains recorded non-controlling interest of $28.9 million and $38.9 million in the Prospector Offshore Fund. For the years ended December 31, 2012, 2011 and 2010 White Mountains recorded $2.6 million, $0.2 million and $(3.2) million of non-controlling interest income (expense) related to the Prospector Offshore Fund. At December 31, 2012, the net amount of capital at risk is equal to White Mountains’ investment in the Fund of $71.1 million, which represents White Mountains’ ownership interest of 71.1% in the Prospector Fund. Tuckerman Fund I On December 31, 2011, the Tuckerman Capital, LP fund (“Tuckerman Fund I”) was dissolved and all of the net assets of the fund were distributed to the owners of the fund, of which White Mountains owned approximately 94%. In conjunction with the dissolution, White Mountains received a portion of the shares of Hamer and Bri-Mar, two small manufacturing companies that were owned by the Tuckerman Fund I. The consolidated results of Hamer and Bri-Mar are included in the Other Operations segment from January 1, 2012 through September 30, 2012, at which point the results of these companies were no longer consolidated by White Mountains. Prior to the dissolution, White Mountains had determined that Tuckerman Fund I was a VIE for which White Mountains was the primary beneficiary and was required to consolidate Tuckerman Fund I. At December 31, 2011, White Mountains consolidated assets of $17.6 million, liabilities of $9.9 million, non-controlling interests of $3.5 million and $2.2 million of non-controlling interest expense related to the companies distributed by Tuckerman Fund I. At December 31, 2010, White Mountains consolidated total assets of $19.4 million and total liabilities of $12.3 million of Tuckerman Fund I. In addition, at December 31, 2010, White Mountains recorded non-controlling interest of $2.4 million in Tuckerman Fund I. For the year ended December 31, 2010 White Mountains recorded $1.0 million of non-controlling interest expense related to Tuckerman Fund I.

Variable Interest Entities

BAM

BAM is a newly formed mutual insurance company. As a mutual company BAM is owned by its members and a portion of each member’s charges represents a contribution to member’s surplus. During 2012, White Mountains capitalized HG Global to fund BAM through the purchase of $503.0 million of BAM Surplus Notes. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional subordinated financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a variable interest entity (“VIE”). The proceeds from the issuance of the BAM Surplus Notes represent substantially all of the equity at risk in BAM at December 31, 2012.

BAM and HG Global, through its wholly-owned subsidiary, HG Re, entered into a first loss reinsurance treaty (“FLRT”), under which HG Re will provide first loss protection up to 15% of par outstanding on each bond insured by BAM in exchange for 60% of the premium, net of a ceding commission, charged by BAM. HG Re's obligations under the FLRT are satisfied by the assets in two collateral trusts: a Regulation 114 Trust and a Supplemental Trust. Losses required to be reimbursed to BAM by HG Re are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, HG Global has the right to designate two directors for election to BAM’s board of directors. White Mountains is required to consolidate the results of BAM. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests.

Reciprocals

Reciprocals are policyholder-owned insurance carriers organized as unincorporated associations. Each policyholder insured by the reciprocal shares risk with the other policyholders. Policyholders share profits and losses in the same proportion as the amount of insurance purchased but are not subject to assessment for net losses of the reciprocal.

OneBeacon had capitalized three reciprocals by loaning funds to them in exchange for surplus notes. In 2002, OneBeacon formed New Jersey Skylands Management LLC (“NJSM”) to provide management services for a fee to New Jersey Skylands Insurance Association, a reciprocal, and its wholly-owned subsidiary New Jersey Skylands Insurance Company (together, “New Jersey Skylands Insurance”). In 2004, OneBeacon formed Houston General Management Company to provide management services for a fee to another reciprocal, Houston General Insurance. In 2006, Adirondack AIF, LLC (“AAIF”), a wholly-owned subsidiary of OneBeacon, entered into an agreement to provide management services for a fee to Adirondack Insurance, a reciprocal. OneBeacon has no ownership interest in the reciprocals. Under the provisions of ASC 810, OneBeacon had determined that each of the reciprocals qualifies as a VIE. Further, OneBeacon had determined that it is the primary beneficiary as it has both the power to direct the activities of the VIE that most significantly impact the entity’s economic performance and the obligation to absorb losses or receive benefits of the entity that could potentially be significant to the VIE as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates Houston General Insurance and, until the completion of the Personal Lines Transaction on July 1, 2010, consolidated New Jersey Skylands Insurance and Adirondack Insurance.

As described in Note 2, the Personal Lines Transaction, which was completed on July 1, 2010, included the sale of NJSM and AAIF and the transfer of the surplus notes issued by New Jersey Skylands Insurance and Adirondack Insurance. Completion of the Personal Lines Transaction triggered deconsolidation of New Jersey Skylands Insurance and Adirondack Insurance.

Subsequent to the Personal Lines Transaction, Houston General Insurance is OneBeacon’s only reciprocal. At December 31, 2012 and 2011, consolidated amounts related to Houston General Insurance included total assets of $97.7 million and $105.9 million, respectively, and total liabilities of $120.4 million and $126.2 million, respectively. At December 31, 2012, the net amount of capital at risk is equal to the surplus note of $23.7 million less the accumulated losses of $22.7 million which includes accrued interest on the surplus note of $19.7 million which eliminates in consolidation.

Prospector Offshore Fund

White Mountains has determined that the Prospector Offshore Fund, Ltd. (“the Prospector Fund”) is a VIE for which White Mountains is the primary beneficiary and is required to consolidate the Prospector Fund. At December 31, 2012 and 2011, White Mountains consolidated total assets of $151.0 million and $135.8 million and total liabilities of $51.1 million and $31.6 million of the Prospector Offshore Fund. In addition, at December 31, 2012 and 2011, White Mountains recorded non-controlling interest of $28.9 million and $38.9 million in the Prospector Offshore Fund. For the years ended December 31, 2012, 2011 and 2010 White Mountains recorded $2.6 million, $0.2 million and $(3.2) million of non-controlling interest income (expense) related to the Prospector Offshore Fund. At December 31, 2012, the net amount of capital at risk is equal to White Mountains’ investment in the Fund of $71.1 million, which represents White Mountains’ ownership interest of 71.1% in the Prospector Fund.

Tuckerman Fund I

On December 31, 2011, the Tuckerman Capital, LP fund (“Tuckerman Fund I”) was dissolved and all of the net assets of the fund were distributed to the owners of the fund, of which White Mountains owned approximately 94%. In conjunction with the dissolution, White Mountains received a portion of the shares of Hamer and Bri-Mar, two small manufacturing companies that were owned by the Tuckerman Fund I. The consolidated results of Hamer and Bri-Mar are included in the Other Operations segment from January 1, 2012 through September 30, 2012, at which point the results of these companies were no longer consolidated by White Mountains.

Prior to the dissolution, White Mountains had determined that Tuckerman Fund I was a VIE for which White Mountains was the primary beneficiary and was required to consolidate Tuckerman Fund I. At December 31, 2011, White Mountains consolidated assets of $17.6 million, liabilities of $9.9 million, non-controlling interests of $3.5 million and $2.2 million of non-controlling interest expense related to the companies distributed by Tuckerman Fund I. At December 31, 2010, White Mountains consolidated total assets of $19.4 million and total liabilities of $12.3 million of Tuckerman Fund I. In addition, at December 31, 2010, White Mountains recorded non-controlling interest of $2.4 million in Tuckerman Fund I. For the year ended December 31, 2010 White Mountains recorded $1.0 million of non-controlling interest expense related to Tuckerman Fund I.