Income Taxes	9 Months Ended
Sep. 30, 2017
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 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, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three and nine months ended September 30, 2017 represented effective tax rates of (47.6)% and (203.9)%. The effective tax rates were different from the U.S. statutory rate of 35%, primarily due to a full valuation allowance on all U.S. operations, a tax benefit recorded at BAM and consolidated pre-tax income being near break-even. For the three and nine months ended September 30, 2017, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2017, BAM recorded a tax benefit of $2.1 million and $6.9 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity. White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2016 represented effective tax rates of (123.9)% and (31.2)%. The effective tax rates were different from the U.S. statutory rate of 35%, primarily due to a tax benefit recorded at BAM and a $14.0 million tax benefit recognized in continuing operations related to the reversal of a valuation allowance that resulted from income that was recognized within discontinued operations. ASC 740 includes an exception to the general principle of intra-period tax allocations that requires a company with a current period loss from continuing operations to consider income recorded in other categories, including discontinued operations, in determining the tax benefit that is allocated to continuing operations. The valuation allowance reversal relates to a consolidated tax group within White Mountains that has a current period loss within continuing operations. Accordingly, the tax benefit resulting from the valuation allowance reversal was recorded in continuing operations with an offsetting tax expense for the same amount in discontinued operations. For the three and nine months ended September 30, 2016, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2016, BAM recorded a tax benefit of $3.3 million and $8.2 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity. In arriving at the effective tax rate for the three and nine months ended September 30, 2017 and 2016, 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, 2017 and 2016. 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. In the second quarter of 2016, White Mountains recorded an increase in deferred tax assets of $0.6 million and a corresponding increase in valuation allowance of $0.6 million related to the settlement of the IRS audit of Guilford Holdings, Inc. and subsidiaries for tax year 2012. 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. In the event 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, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States.

White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three and nine months ended September 30, 2017 represented effective tax rates of (47.6)% and (203.9)%. The effective tax rates were different from the U.S. statutory rate of 35%, primarily due to a full valuation allowance on all U.S. operations, a tax benefit recorded at BAM and consolidated pre-tax income being near break-even. For the three and nine months ended September 30, 2017, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2017, BAM recorded a tax benefit of $2.1 million and $6.9 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity.

White Mountains’s income tax benefit related to pre-tax loss from continuing operations for the three and nine months ended September 30, 2016 represented effective tax rates of (123.9)% and (31.2)%. The effective tax rates were different from the U.S. statutory rate of 35%, primarily due to a tax benefit recorded at BAM and a $14.0 million tax benefit recognized in continuing operations related to the reversal of a valuation allowance that resulted from income that was recognized within discontinued operations. ASC 740 includes an exception to the general principle of intra-period tax allocations that requires a company with a current period loss from continuing operations to consider income recorded in other categories, including discontinued operations, in determining the tax benefit that is allocated to continuing operations. The valuation allowance reversal relates to a consolidated tax group within White Mountains that has a current period loss within continuing operations. Accordingly, the tax benefit resulting from the valuation allowance reversal was recorded in continuing operations with an offsetting tax expense for the same amount in discontinued operations. For the three and nine months ended September 30, 2016, BAM had amounts recorded in shareholders’ equity related to member surplus contributions that were available to partially offset its loss from continuing operations. As a result, for the three and nine months ended September 30, 2016, BAM recorded a tax benefit of $3.3 million and $8.2 million in net income from continuing operations, with an offsetting amount recorded in shareholders’ equity.

In arriving at the effective tax rate for the three and nine months ended September 30, 2017 and 2016, 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, 2017 and 2016.

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.

In the second quarter of 2016, White Mountains recorded an increase in deferred tax assets of $0.6 million and a corresponding increase in valuation allowance of $0.6 million related to the settlement of the IRS audit of Guilford Holdings, Inc. and subsidiaries for tax year 2012.

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.