Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2020
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 Barbados, Ireland, Israel, Luxembourg, the United Kingdom and the United States. In the first quarter of 2020, White Mountains adopted ASU 2019-12, Simplifying the Accounting for Income Taxes (ASC 740) (“ASU 2019-12”). For periods subsequent to the adoption of ASU 2019-12, White Mountains records both the tax expense related to BAM’s MSC and the related valuation allowance on such taxes through the non-controlling interest equity. Prior to the adoption of ASU 2019-12, White Mountains recorded the tax expense related to BAM’s MSC directly to non-controlling interest equity, while the valuation allowance on such taxes was recorded through the income statement. 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 (“Guilford”) 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 consisted of withholding taxes and the establishment of a partial valuation allowance on deferred tax assets of various service companies, other entities and investments that are included in the Other Operations segment. White Mountains’s income tax expense related to pre-tax income from continuing operations for the three and nine months ended September 30, 2019 represented an effective tax rate of 17.2% and 5.4%. 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, state income taxes and a tax benefit recorded at BAM. Also, the effective tax rate for the nine months ended September 30, 2019 was different from the U.S. statutory rate of 21.0% due to the release of a full valuation allowance on the net deferred tax assets of Guilford. Guilford includes Kudu, its investment in MediaAlpha, various service companies and certain other entities and investments that are included in the Other Operations segment. For BAM, member surplus contributions (“MSC”) and the related taxes thereon were recorded directly to non-controlling interest equity, while the valuation allowance on such taxes was recorded through the income statement. For the three and nine months ended September 30, 2019, BAM recorded a tax benefit of $3.6 million and $6.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. In arriving at the effective tax rate for the three and nine months ended September 30, 2020 and 2019, 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, 2020 and 2019. 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 2013.

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.