Income Taxes	3 Months Ended
Mar. 31, 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, Ireland, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States. White Mountains’ income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2017, represented a net effective tax rate of 10.3%. The effective tax rate for the three months ended March 31, 2017 was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than 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 (114.1)%. 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%. In arriving at the effective tax rate for the three months ended March 31, 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. 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. In the first and second quarters of 2016 White Mountains recorded tax benefits of $12.8 million and $3.5 million respectively related to the settlement of IRS audits of certain subsidiaries of OneBeacon for tax years 2007-2009 and 2010-2012. 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, Ireland, Israel, Luxembourg, the Netherlands, the United Kingdom and the United States.

White Mountains’ income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2017, represented a net effective tax rate of 10.3%. The effective tax rate for the three months ended March 31, 2017 was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than 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 (114.1)%. 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%.

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

In the first and second quarters of 2016 White Mountains recorded tax benefits of $12.8 million and $3.5 million respectively related to the settlement of IRS audits of certain subsidiaries of OneBeacon for tax years 2007-2009 and 2010-2012.

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.