Contingencies	6 Months Ended
Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following description presents significant legal contingencies, ongoing non-claims related litigation or arbitration as of June 30, 2018: Sirius Group Tax Contingencies A subsidiary of Sirius Group, which was sold by White Mountains in 2016, has been denied interest deductions by the Swedish Tax Authority (“STA”) for tax years 2013-2016. The Swedish subsidiary has filed an appeal in the Swedish Administrative Court for tax year 2013 and will preserve its right to contest the STA’s decision for later years if needed. White Mountains believes that Sirius Group’s Swedish subsidiary will prevail in the court proceedings. However, if the ultimate decision in the Swedish courts is unfavorable, White Mountains would be required to indemnify Sirius Group for the tax effect of the interest deductions, which is approximately $17.3 million. NSM Contingent Liabilities In connection with White Mountains’s acquisition of NSM, White Mountains and NSM entered into an agreement with AIG to facilitate a sale of NSM’s U.S. collector car renewal rights owned by AIG to a third party by December 31, 2019. Under the terms of the agreement, if White Mountains and NSM are unable to facilitate a sale by December 31, 2019, AIG has the right to require NSM to purchase the renewal rights for $82.5 million. The Company has guaranteed NSM’s obligations under the agreement with AIG. The manner in which these obligations are ultimately discharged depends on a number of factors, including the market value of the renewal rights, the number of potential buyers and the current and prospective environment for U.S. collector car insurance. White Mountains believes that the estimated fair value of the renewal rights is greater than $82.5 million and, accordingly, no accrual of a liability is necessary at June 30, 2018.

Contingencies

Legal Contingencies

White Mountains is subject to litigation and arbitration in the normal course of business. White Mountains considers the requirements of ASC 450 when evaluating its exposure to litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. White Mountains does not have any current litigation that may have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows.

The following description presents significant legal contingencies, ongoing non-claims related litigation or arbitration as of June 30, 2018:

Sirius Group Tax Contingencies

A subsidiary of Sirius Group, which was sold by White Mountains in 2016, has been denied interest deductions by the Swedish Tax Authority (“STA”) for tax years 2013-2016. The Swedish subsidiary has filed an appeal in the Swedish Administrative Court for tax year 2013 and will preserve its right to contest the STA’s decision for later years if needed. White Mountains believes that Sirius Group’s Swedish subsidiary will prevail in the court proceedings. However, if the ultimate decision in the Swedish courts is unfavorable, White Mountains would be required to indemnify Sirius Group for the tax effect of the interest deductions, which is approximately $17.3 million.

NSM Contingent Liabilities

In connection with White Mountains’s acquisition of NSM, White Mountains and NSM entered into an agreement with AIG to facilitate a sale of NSM’s U.S. collector car renewal rights owned by AIG to a third party by December 31, 2019. Under the terms of the agreement, if White Mountains and NSM are unable to facilitate a sale by December 31, 2019, AIG has the right to require NSM to purchase the renewal rights for $82.5 million. The Company has guaranteed NSM’s obligations under the agreement with AIG. The manner in which these obligations are ultimately discharged depends on a number of factors, including the market value of the renewal rights, the number of potential buyers and the current and prospective environment for U.S. collector car insurance. White Mountains believes that the estimated fair value of the renewal rights is greater than $82.5 million and, accordingly, no accrual of a liability is necessary at June 30, 2018.