Income Taxes	3 Months Ended
Mar. 31, 2016
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 Barbados, Germany, Gibraltar, Israel, Luxembourg, the Netherlands, Peru, Sweden, Switzerland, the United Kingdom and the United States. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended March 31, 2016, represented a net effective tax rate of (75.7)%. As noted below, the effective tax rate for the three months ended March 31, 2016 was impacted by a $12.8 million tax benefit on the settlement of the 2007-2009 IRS exam. Without the tax benefit on the settlement of the 2007-2009 IRS exam there would have been tax expense, which would have resulted in a net effective tax rate that is approximately the same as the U.S. statutory rate of 35%. White Mountains’s effective tax rate for the three months ended March 31, 2015 was not meaningful as pre-tax income was near break-even. In arriving at the effective tax rate for the three months ended March 31, 2016 and 2015, 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, 2016 and 2015. 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. 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 2010. On January 19, 2016, White Mountains received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Overassessment) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved with the Joint Committee on Taxation. As the receipt of the Form 870-AD described above represents formal settlement, OneBeacon recorded a tax benefit of $12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009. 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. On March 22, 2016, White Mountains received Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2010, 2011 and 2012. The estimated total overpayment is $6.0 million. However, $4.7 million of the adjustments relate to items for which the expense deduction has been disallowed in an earlier period, but deductibility will occur in a year being examined. Because of the impact of deferred tax accounting, other than interest and penalties, the allowance of the deduction in the exam period would not affect the effective tax rate. 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 Barbados, Germany, Gibraltar, Israel, Luxembourg, the Netherlands, Peru, Sweden, Switzerland, the United Kingdom and the United States.

White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended March 31, 2016, represented a net effective tax rate of (75.7)%. As noted below, the effective tax rate for the three months ended March 31, 2016 was impacted by a $12.8 million tax benefit on the settlement of the 2007-2009 IRS exam. Without the tax benefit on the settlement of the 2007-2009 IRS exam there would have been tax expense, which would have resulted in a net effective tax rate that is approximately the same as the U.S. statutory rate of 35%.

White Mountains’s effective tax rate for the three months ended March 31, 2015 was not meaningful as pre-tax income was near break-even.

In arriving at the effective tax rate for the three months ended March 31, 2016 and 2015, 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, 2016 and 2015.

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.

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 2010.

On January 19, 2016, White Mountains received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Overassessment) from the IRS Appeals Office relating to the examination of tax years 2007, 2008 and 2009 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved with the Joint Committee on Taxation. As the receipt of the Form 870-AD described above represents formal settlement, OneBeacon recorded a tax benefit of $12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009.

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. On March 22, 2016, White Mountains received Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2010, 2011 and 2012. The estimated total overpayment is $6.0 million. However, $4.7 million of the adjustments relate to items for which the expense deduction has been disallowed in an earlier period, but deductibility will occur in a year being examined. Because of the impact of deferred tax accounting, other than interest and penalties, the allowance of the deduction in the exam period would not affect the effective tax rate. 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.