Contingencies	3 Months Ended
Mar. 31, 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 March 31, 2018: Sirius 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. Sirius Group believes it is more likely than not that it 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 $18.5 million.

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 March 31, 2018:

Sirius 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. Sirius Group believes it is more likely than not that it 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 $18.5 million.