Variable Interest Entities	12 Months Ended
Dec. 31, 2017
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 writes. A portion of the premium is a member’s surplus contribution (“MSC”) and the remainder is a risk premium. In the event of a municipal bond refunding, the MSC from the original issuance can be reutilized, in effect serving as a credit against the total insurance premium on the refunding of the municipal bond. Issuers of debt insured by BAM are members of BAM so long as any of their BAM-insured debt is outstanding, and as members they have certain interests in BAM, including the right to vote for BAM’s directors and to receive dividends in the future, if declared. The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE. At inception, BAM and HG Re also entered into the FLRT. HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. HG Re’s obligations under the FLRT are satisfied by the assets in the Regulation 114 Trust and the Supplemental Trust. Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, under the FLRT, HG Holdings Ltd, a subsidiary of HG Global, has the right to designate two directors for election to BAM’s board of directors. Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. Prospector Offshore Fund In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. (the “Prospector Offshore Fund”) 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 writes. A portion of the premium is a member’s surplus contribution (“MSC”) and the remainder is a risk premium. In the event of a municipal bond refunding, the MSC from the original issuance can be reutilized, in effect serving as a credit against the total insurance premium on the refunding of the municipal bond. Issuers of debt insured by BAM are members of BAM so long as any of their BAM-insured debt is outstanding, and as members they have certain interests in BAM, including the right to vote for BAM’s directors and to receive dividends in the future, if declared.

The equity at risk funded by BAM’s members is not sufficient to fund its operations without the additional financial support provided by the BAM Surplus Notes and accordingly, BAM is considered to be a VIE.

At inception, BAM and HG Re also entered into the FLRT. HG Re provides first loss protection up to 15%-of-par outstanding on each municipal bond insured by BAM. In return, BAM cedes 60% of the risk premium charged for insuring the municipal bond, net of a ceding commission. HG Re’s obligations under the FLRT are satisfied by the assets in the Regulation 114 Trust and the Supplemental Trust. Losses required to be reimbursed under the FLRT are subject to an aggregate limit equal to the assets held in the collateral trusts at any point in time. In addition, under the FLRT, HG Holdings Ltd, a subsidiary of HG Global, has the right to designate two directors for election to BAM’s board of directors.

Since BAM is owned by its members, its equity and results of operations are included in non-controlling interests. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary.

Prospector Offshore Fund

In 2015, White Mountains redeemed its interest in the Prospector Offshore Fund, Ltd. (the “Prospector Offshore Fund”) 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.