Variable Interest Entities	12 Months Ended
Dec. 31, 2016
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	Variable Interest Entities BAM As a mutual insurance company, BAM is owned by its members. BAM charges an insurance premium on each municipal bond insurance policy it insures, a portion of which is contributed to BAM’s qualified statutory capital and conveys to the issuer certain interests in BAM. During 2012, White Mountains formed and capitalized HG Global to fund the startup of 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 VIE. 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. Houston General Insurance Exchange Houston General Management Company, a wholly-owned indirect subsidiary of OneBeacon provides management services for a fee to a reciprocal, HGIE. OneBeacon holds HGIE Surplus Notes, the remaining balance of which is $5.0 million as of December 31, 2016. The principal and interest on the HGIE Surplus Notes is repayable to OneBeacon only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. OneBeacon has no ownership interest in the reciprocal. OneBeacon has determined that HGIE qualifies as a VIE. Furthermore, OneBeacon has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates HGIE. Consolidated amounts related to HGIE included total assets of $5.1 million and total liabilities of $5.0 million as of both December 31, 2016 and 2015. As of December 31, 2016, the net amount of capital at risk is equal to the HGIE Surplus Notes of $5.0 million less the accumulated losses of $0.5 million which includes accrued interest on the HGIE Surplus Notes of $0.6 million that eliminates in consolidation. Star & Shield Insurance Exchange SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE. Furthermore, White Mountains has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal. Thus, SSIE qualifies as a VIE and is required to consolidate SSIE in its GAAP financial statements. As of December 31, 2015, White Mountains held $21.0 million of SSIE Surplus Notes. On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in the SSIE Surplus Notes which resulted in a $21.0 million decrease to net income attributable to White Mountains’s common shareholders and a corresponding increase to net income attributable to non-controlling interests. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to National General Holdings Corp., which was subsequently terminated on December 2, 2016. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services LLC. Consequently, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015. As of December 31, 2016 and 2015, SSIE had total assets of $9.2 million and $14.2 million and total liabilities of $4.8 million and $30.3 million. Prospector Offshore Fund In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. of which White Mountains’s owned 67.6% prior to the redemption on June 30, 2015. The consolidated results of the Prospector Offshore Fund are included in Other Operations from January 1, 2015 through June 30, 2015, at which point the results of the Prospector Offshore Fund were no longer consolidated by White Mountains. Prior to the redemption, White Mountains determined that the Prospector Offshore Fund was a VIE of which White Mountains was the primary beneficiary and was required to consolidate.

Variable Interest Entities

BAM

As a mutual insurance company, BAM is owned by its members. BAM charges an insurance premium on each municipal bond insurance policy it insures, a portion of which is contributed to BAM’s qualified statutory capital and conveys to the issuer certain interests in BAM. During 2012, White Mountains formed and capitalized HG Global to fund the startup of 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 VIE.

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.

Houston General Insurance Exchange

Houston General Management Company, a wholly-owned indirect subsidiary of OneBeacon provides management services for a fee to a reciprocal, HGIE. OneBeacon holds HGIE Surplus Notes, the remaining balance of which is $5.0 million as of December 31, 2016. The principal and interest on the HGIE Surplus Notes is repayable to OneBeacon only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. OneBeacon has no ownership interest in the reciprocal.

OneBeacon has determined that HGIE qualifies as a VIE. Furthermore, OneBeacon has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates HGIE.

Consolidated amounts related to HGIE included total assets of $5.1 million and total liabilities of $5.0 million as of both December 31, 2016 and 2015. As of December 31, 2016, the net amount of capital at risk is equal to the HGIE Surplus Notes of $5.0 million less the accumulated losses of $0.5 million which includes accrued interest on the HGIE Surplus Notes of $0.6 million that eliminates in consolidation.

Star & Shield Insurance Exchange

SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE. Furthermore, White Mountains has determined that it is the primary beneficiary as a result of the management services provided to the reciprocal. Thus, SSIE qualifies as a VIE and is required to consolidate SSIE in its GAAP financial statements.

As of December 31, 2015, White Mountains held $21.0 million of SSIE Surplus Notes. On July 1, 2016, SSIE voluntarily ceased writing new policies. As a result, White Mountains wrote off its investment in the SSIE Surplus Notes which resulted in a $21.0 million decrease to net income attributable to White Mountains’s common shareholders and a corresponding increase to net income attributable to non-controlling interests.

On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to National General Holdings Corp., which was subsequently terminated on December 2, 2016. On January 13, 2017, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE Surplus Notes to K2 Insurance Services LLC. Consequently, White Mountains has presented Star & Shield’s and SSIE’s assets and liabilities as held for sale as of December 31, 2016 and 2015. As of December 31, 2016 and 2015, SSIE had total assets of $9.2 million and $14.2 million and total liabilities of $4.8 million and $30.3 million.

Prospector Offshore Fund

In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. of which White Mountains’s owned 67.6% prior to the redemption on June 30, 2015. The consolidated results of the Prospector Offshore Fund are included in Other Operations from January 1, 2015 through June 30, 2015, at which point the results of the Prospector Offshore Fund were no longer consolidated by White Mountains. Prior to the redemption, White Mountains determined that the Prospector Offshore Fund was a VIE of which White Mountains was the primary beneficiary and was required to consolidate.