Commitments and Contingencies	12 Months Ended
Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office spaces under non-cancellable operating leases that expire on various dates through 2022. Rental expense for all of White Mountains’s locations was $5.5 million, $3.5 million and $3.4 million for the years ended December 31, 2018, 2017 and 2016. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $6.6 million, $4.9 million, $4.2 million, and $12.0 million for the years 2019, 2020, 2021 and 2022 and thereafter. White Mountains also has future binding commitments to fund certain other long-term investments. These commitments, which totaled $170.2 million as of December 31, 2018, do not have fixed funding dates. 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 December 31, 2018: Esurance On October 7, 2011, the Company completed the sale of its Esurance Holdings, Inc. and its subsidiaries and Answer Financial Inc. and its subsidiaries (collectively, “Esurance”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011. Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) losses of Esurance arising from extra-contractual claims and claims in excess of policy limits, (b) certain corporate reorganizations effected to remove entities from Esurance that were not being sold in the transaction, and (c) certain tax matters, including certain net operating losses being less than stated levels. Sirius Tax Contingency 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. In 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 connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, for the year ended December 31, 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 has appealed 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 American International Group, Inc. (“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 equal to or greater than $82.5 million and, accordingly, no accrual of a liability is necessary at December 31, 2018.

Commitments and Contingencies

White Mountains leases certain office spaces under non-cancellable operating leases that expire on various dates through 2022. Rental expense for all of White Mountains’s locations was $5.5 million, $3.5 million and $3.4 million for the years ended December 31, 2018, 2017 and 2016. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $6.6 million, $4.9 million, $4.2 million, and $12.0 million for the years 2019, 2020, 2021 and 2022 and thereafter.

White Mountains also has future binding commitments to fund certain other long-term investments. These commitments, which totaled $170.2 million as of December 31, 2018, do not have fixed funding dates.

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 December 31, 2018:

Esurance

On October 7, 2011, the Company completed the sale of its Esurance Holdings, Inc. and its subsidiaries and Answer Financial Inc. and its subsidiaries (collectively, “Esurance”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011. Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) losses of Esurance arising from extra-contractual claims and claims in excess of policy limits, (b) certain corporate reorganizations effected to remove entities from Esurance that were not being sold in the transaction, and (c) certain tax matters, including certain net operating losses being less than stated levels.

Sirius Tax Contingency

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. In 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 connection with the sale, White Mountains indemnified Sirius Group against the loss of certain tax attributes, including those related to these interest deductions. As a result, for the year ended December 31, 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 has appealed 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 American International Group, Inc. (“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 equal to or greater than $82.5 million and, accordingly, no accrual of a liability is necessary at December 31, 2018.