Income Taxes	9 Months Ended
Sep. 30, 2015
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 consolidated subsidiaries and branches are subject to tax are Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Israel, Luxembourg, Malaysia, the Netherlands, Peru, Singapore, Sweden, Switzerland, the United Kingdom and the United States. White Mountains’s income tax benefit from continuing operations for the third quarter of 2015 was near zero and White Mountains’s income tax benefit from continuing operations for the third quarter of 2014 represented an effective tax rate of 13.0%. White Mountains’s income tax expense from continuing operations for the first nine months of 2015 was near zero and White Mountains's income tax expense from continuing operations for the first nine months of 2014 represented an effective tax rate 31.0%. The effective tax rates for the nine months ended September 30, 2015 and the three and nine months ended 2014 were lower than the U.S. statutory rate of 35% due to income generated in jurisdictions with lower tax rates than the United States. In arriving at the effective tax rate for the three and nine months ended September 30, 2015 and 2014, 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, 2015 and 2014. 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. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense. White Mountains is no longer subject to U.S. federal or state tax examinations by tax authorities for years before 2007. With few exceptions, White Mountains is no longer subject to non-U.S. income tax examinations by tax authorities for years before 2005. On July 28, 2011, the IRS commenced an examination of the income tax returns for 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon. On September 1, 2015, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009. The estimated total overpayment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $4.0 million. However, $2.7 million of the proposed overpayment relates to items for which the expense deduction was disallowed in an earlier year and is being allowed in a year being examined. White Mountains agrees with the adjustments proposed by the IRS and is awaiting final review by the Joint Committee on Taxation. Although the timing of the resolution of these issues is uncertain, it is reasonably possible that the resolution could occur within the next twelve months. When ultimately settled, White Mountains does not expect the resolution of this examination to result in a material adverse change to its financial position results of operations and cash flows. On September 5, 2013, the IRS commenced an examination of the income tax returns for 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows. On December 18, 2014, the IRS commenced an examination of the 2012 income tax return for Guilford Holdings, Inc. and subsidiaries. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.

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 consolidated subsidiaries and branches are subject to tax are Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Israel, Luxembourg, Malaysia, the Netherlands, Peru, Singapore, Sweden, Switzerland, the United Kingdom and the United States.

White Mountains’s income tax benefit from continuing operations for the third quarter of 2015 was near zero and White Mountains’s income tax benefit from continuing operations for the third quarter of 2014 represented an effective tax rate of 13.0%. White Mountains’s income tax expense from continuing operations for the first nine months of 2015 was near zero and White Mountains's income tax expense from continuing operations for the first nine months of 2014 represented an effective tax rate 31.0%. The effective tax rates for the nine months ended September 30, 2015 and the three and nine months ended 2014 were lower than the U.S. statutory rate of 35% due to income generated in jurisdictions with lower tax rates than the United States.

In arriving at the effective tax rate for the three and nine months ended September 30, 2015 and 2014, 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, 2015 and 2014.

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. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense.

White Mountains is no longer subject to U.S. federal or state tax examinations by tax authorities for years before 2007. With few exceptions, White Mountains is no longer subject to non-U.S. income tax examinations by tax authorities for years before 2005.

On July 28, 2011, the IRS commenced an examination of the income tax returns for 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon. On September 1, 2015, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009. The estimated total overpayment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $4.0 million. However, $2.7 million of the proposed overpayment relates to items for which the expense deduction was disallowed in an earlier year and is being allowed in a year being examined. White Mountains agrees with the adjustments proposed by the IRS and is awaiting final review by the Joint Committee on Taxation. Although the timing of the resolution of these issues is uncertain, it is reasonably possible that the resolution could occur within the next twelve months. When ultimately settled, White Mountains does not expect the resolution of this examination to result in a material adverse change to its financial position results of operations and cash flows.

On September 5, 2013, the IRS commenced an examination of the income tax returns for 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.

On December 18, 2014, the IRS commenced an examination of the 2012 income tax return for Guilford Holdings, Inc. and subsidiaries. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.