Income Taxes	3 Months Ended
Mar. 31, 2018
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. If there is a change in the current law such that 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, Gibraltar, 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 months ended March 31, 2018 represented an effective tax rate of (1.1)%. The effective tax rate was different from the U.S. statutory rate of 21%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations, withholding taxes and a tax benefit recorded at BAM. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2018, BAM recorded a tax benefit of $0.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three months ended March 31, 2017 represented an effective tax rate of 8.3%. The effective tax rate was different from the U.S. statutory rate of 35%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations and a tax benefit recorded at BAM. For BAM, MSC and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2017, BAM recorded a tax benefit of $2.7 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 months ended March 31, 2018 and 2017, 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, 2018 and 2017. 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.

Income Taxes

The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. If there is a change in the current law such that 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, Gibraltar, 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 months ended March 31, 2018 represented an effective tax rate of (1.1)%. The effective tax rate was different from the U.S. statutory rate of 21%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations, withholding taxes and a tax benefit recorded at BAM. For BAM, member surplus contributions (“MSC”) and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2018, BAM recorded a tax benefit of $0.6 million associated with the valuation allowance on taxes related to MSC that is included in the effective tax rate.

White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three months ended March 31, 2017 represented an effective tax rate of 8.3%. The effective tax rate was different from the U.S. statutory rate of 35%, primarily due to a full valuation allowance on all net deferred tax assets at U.S. operations and a tax benefit recorded at BAM. For BAM, MSC and the related taxes thereon are recorded directly to non-controlling interest equity, while the valuation allowance on such taxes is recorded through the income statement. For the three months ended March 31, 2017, BAM recorded a tax benefit of $2.7 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 months ended March 31, 2018 and 2017, 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, 2018 and 2017.

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.