Contingencies	9 Months Ended
Sep. 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 September 30, 2018: Sirius Group Tax Contingency In late October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI in 2016. In connection with the sale, White Mountains indemnified Sirius Group against the loss of these interest deductions. As a result, in the third quarter of 2018, White Mountains recorded a loss of $17.3 million within net (loss) gain on sale of discontinued operations reflecting the value of these interest deductions. Sirius Group intends to appeal the decision to the Swedish Administrative Court of Appeal. NSM Contingent Liability 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 September 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 September 30, 2018:

Sirius Group Tax Contingency

In late October 2018, the Swedish Administrative Court ruled against Sirius Group on its appeal of the Swedish Tax Agency’s denial of certain interest deductions relating to periods prior to the sale of Sirius Group to CMI in 2016. In connection with the sale, White Mountains indemnified Sirius Group against the loss of these interest deductions. As a result, in the third quarter of 2018, White Mountains recorded a loss of $17.3 million within net (loss) gain on sale of discontinued operations reflecting the value of these interest deductions. Sirius Group intends to appeal the decision to the Swedish Administrative Court of Appeal.

NSM Contingent Liability

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 September 30, 2018.