Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law and taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s subsidiaries and branches are subject to tax are Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax expense related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2021 represented an effective tax rate of (6.0)% and (12.4)%. The effective tax rate was different from the U.S. statutory rate of 21.0%, due to losses in jurisdictions with lower tax rates than the United States, a full valuation allowance on net deferred tax assets in certain U.S. operations, consisting of the WM Adams, Inc. consolidated tax group within the Other Operations segment and BAM, and state income taxes. For the nine months ended September 30, 2021, the effective rate was also different from the U.S. statutory rate of 21.0% due to additional tax expense related to the revaluation of U.K. deferred tax assets and liabilities. On June 10, 2021, the U.K. enacted an increase in its corporate tax rate from 19.0% to 25.0% for periods after April 1, 2023. On June 30, 2021, White Mountains increased its net U.K. deferred tax liability to reflect the higher tax rate on temporary differences projected to reverse after the new rate becomes effective. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three and nine months ended September 30, 2020 represented an effective tax rate of 30.7% and 33.7%. The effective tax rate was different from the U.S. statutory rate of 21.0% due to tax expense associated with the reorganization of the Guilford Holdings, Inc. consolidated U.S. tax group in preparation for the MediaAlpha IPO and state income taxes, partially offset by income generated in jurisdictions with lower tax rates than the United States. The additional tax expense associated with the reorganization of the Guilford Holdings, Inc. consolidated U.S. tax group within the Other Operations segment consisted of withholding taxes and the establishment of a partial valuation allowance on deferred tax assets of various service companies, other entities and investments. In arriving at the effective tax rate for the three and nine months ended September 30, 2021 and 2020, White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2021 and 2020. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. With few exceptions, White Mountains is no longer subject to U.S. federal, state, or non-U.S. income tax examinations by tax authorities for years before 2015.

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.