Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2022
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 such that taxes are imposed, the Bermuda Exempted Undertakings Tax Protection Act of 1966 states that the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035. The Company has subsidiaries and branches that operate in various other jurisdictions around the world and are subject to tax in the jurisdictions in which they operate. As of March 31, 2022, the primary jurisdictions in which the Company’s subsidiaries and branches were subject to tax were Ireland, Israel, Luxembourg, the United Kingdom and the United States. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three months ended March 31, 2022 represented an effective tax rate of 31.1%. The effective tax rate was different from the U.S. statutory rate of 21.0%, due to income in jurisdictions with lower tax rates than the United States and a decline in the 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, offset by an increase in the full valuation allowance at BAM and state income taxes. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three months ended March 31, 2021 represented an effective tax rate of 6.9%. The effective tax rate was different from the U.S. statutory rate of 21.0% due to income generated in jurisdictions with lower tax rates than the United States. In arriving at the effective tax rate for the three months ended March 31, 2022 and 2021, 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, 2022 and 2021. 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 2016.

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.