Income Taxes	9 Months Ended
Sep. 30, 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, Gibraltar, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’s effective tax rate for the three months ended September 30, 2016 was (33.5)%. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by 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. White Mountains’s effective tax rate for the nine months ended September 30, 2016 was (84.9)%. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the nine months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by a $3.5 million favorable settlement of the 2010-2012 IRS exam, by a $12.8 million favorable settlement of the 2007-2009 IRS exam and by 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 as described above. 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. White Mountains’s income tax benefit from continuing operations for the first nine months of 2015 was near zero. The effective rates for the three and nine months ended September 30, 2015 represented an effective tax rate that was lower than the U.S. statutory rate of 35% due to income forecasted 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, 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 2012. 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, White Mountains recorded a tax benefit of $12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009. On May 27, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax years 2010, 2011 and 2012 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded a tax benefit of $3.5 million in the second quarter of 2016 related to tax years 2010, 2011 and 2012. On June 22, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax year 2012 for Guilford Holdings, Inc. and subsidiaries. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded an increase to net operating loss carryforwards of $1.7 million related to tax year 2012.

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, Gibraltar, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States.

White Mountains’s effective tax rate for the three months ended September 30, 2016 was (33.5)%. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the three months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by 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.

White Mountains’s effective tax rate for the nine months ended September 30, 2016 was (84.9)%. White Mountains’s income tax benefit related to pre-tax income from continuing operations for the nine months ended September 30, 2016 was different from the U.S. statutory rate of 35% due to changes in forecasted earnings by jurisdiction used in determining interim tax expense. The rate was also impacted by a $3.5 million favorable settlement of the 2010-2012 IRS exam, by a $12.8 million favorable settlement of the 2007-2009 IRS exam and by 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 as described above. 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.

White Mountains’s income tax benefit from continuing operations for the first nine months of 2015 was near zero. The effective rates for the three and nine months ended September 30, 2015 represented an effective tax rate that was lower than the U.S. statutory rate of 35% due to income forecasted 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, 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 2012.

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, White Mountains recorded a tax benefit of $12.8 million in the first quarter of 2016 related to tax years 2007, 2008 and 2009.

On May 27, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax years 2010, 2011 and 2012 for certain U.S. Subsidiaries of OneBeacon. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded a tax benefit of $3.5 million in the second quarter of 2016 related to tax years 2010, 2011 and 2012.

On June 22, 2016, White Mountains received the Closing Letter (Letter 1156) from the IRS Revenue Agent relating to the examination of tax year 2012 for Guilford Holdings, Inc. and subsidiaries. All disputed items have now been agreed to and resolved. As the receipt of the Closing Letter described above represents formal settlement, White Mountains recorded an increase to net operating loss carryforwards of $1.7 million related to tax year 2012.