Variable Interest Entities	12 Months Ended
Dec. 31, 2015
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 and a portion of each member’s policy payments 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 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, Houston General Insurance Exchange (“HGIE”). As of December 31, 2015, OneBeacon holds a surplus note issued by HGIE, the remaining balance of which is $5.0 million as of December 31, 2015. The principal and interest on the surplus note 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. As of December 31, 2015 and 2014, consolidated amounts related to HGIE included total assets of $5.1 million and $2.4 million and total liabilities of $5.0 million and $4.0 million. As of December 31, 2015, the net amount of capital at risk is equal to the surplus note of $5.0 million less the accumulated losses of $0.2 million which includes accrued interest on the surplus note of $0.3 million that eliminates in consolidation. Star & Shield Insurance Exchange Star & Shield Risk Management LLC, a wholly-owned indirect subsidiary of White Mountains provides management and other services for a fee to a reciprocal, Star & Shield Insurance Exchange (“SSIE”). As of December 31, 2015, White Mountains held $21.0 million of surplus notes issued by SSIE. The principal and interest on the surplus notes are repayable to White Mountains 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. Because SSIE is consolidated in White Mountains’s financial statements, the SSIE Surplus Notes and accrued interest are classified as intercompany notes, carried at face value and eliminated in consolidation. White Mountains has no ownership interest in SSIE. White Mountains has determined that SSIE qualifies as a VIE. Furthermore, White Mountains 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, White Mountains consolidates SSIE. As of December 31, 2015 and 2014, consolidated amounts related to SSIE included total assets of $14.2 million and $13.5 million and total liabilities of $30.3 million and $25.9 million. For both December 31, 2015 and 2014, the net amount of capital at risk is equal to the surplus notes of $21.0 million less the accumulated losses of $16.0 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 and a portion of each member’s policy payments 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 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, Houston General Insurance Exchange (“HGIE”). As of December 31, 2015, OneBeacon holds a surplus note issued by HGIE, the remaining balance of which is $5.0 million as of December 31, 2015. The principal and interest on the surplus note 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.

As of December 31, 2015 and 2014, consolidated amounts related to HGIE included total assets of $5.1 million and $2.4 million and total liabilities of $5.0 million and $4.0 million. As of December 31, 2015, the net amount of capital at risk is equal to the surplus note of $5.0 million less the accumulated losses of $0.2 million which includes accrued interest on the surplus note of $0.3 million that eliminates in consolidation.

Star & Shield Insurance Exchange

Star & Shield Risk Management LLC, a wholly-owned indirect subsidiary of White Mountains provides management and other services for a fee to a reciprocal, Star & Shield Insurance Exchange (“SSIE”). As of December 31, 2015, White Mountains held $21.0 million of surplus notes issued by SSIE. The principal and interest on the surplus notes are repayable to White Mountains 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. Because SSIE is consolidated in White Mountains’s financial statements, the SSIE Surplus Notes and accrued interest are classified as intercompany notes, carried at face value and eliminated in consolidation. White Mountains has no ownership interest in SSIE.

White Mountains has determined that SSIE qualifies as a VIE. Furthermore, White Mountains 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, White Mountains consolidates SSIE.

As of December 31, 2015 and 2014, consolidated amounts related to SSIE included total assets of $14.2 million and $13.5 million and total liabilities of $30.3 million and $25.9 million. For both December 31, 2015 and 2014, the net amount of capital at risk is equal to the surplus notes of $21.0 million less the accumulated losses of $16.0 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.