Income Taxes	9 Months Ended
Sep. 30, 2012
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 Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Luxembourg, the Netherlands, Singapore, Sweden, Switzerland, the United Kingdom and the United States. White Mountains' income tax expense for the three months ended September 30, 2012 represented an effective tax rate of 35.4%, which is in line with the U.S. statutory rate of 35%. White Mountains’ income tax expense for the nine months ended September 30, 2012 represented effective tax rate of 27.2%, which differed from the U.S. statutory rate of 35% primarily due to income generated in jurisdictions other than the United States. White Mountains' effective tax rate for the three and nine months ended September 30, 2011 was not meaningful as pre-tax income was near break-even. In arriving at the effective tax rate for the three and nine months ended September 30, 2012 and 2011, White Mountains forecasted the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2012 and 2011 and included these gains (losses) in the effective tax rate calculation pursuant to ASC 740-270. 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 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. During the next twelve months, it is possible that certain planning strategies will no longer be sufficient to utilize the entire deferred tax asset, which could result in material changes to White Mountains’ deferred tax assets and tax expense. Upon completion of the Runoff Transaction, it is expected that the unrecognized tax benefits associated with tax positions where the deductibility is certain but the timing is uncertain, will decrease by approximately $11.5 million. White Mountains does not expect the decrease to result in a material change to its financial position. 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 2005. The IRS is conducting an examination of income tax returns for 2005 and 2006 for certain U.S. subsidiaries of OneBeacon. On January 5, 2011, White Mountains received Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2005 and 2006. The estimated total assessment, including interest and utilization of alternative minimum and foreign tax credit carryovers, is $20.6 million. White Mountains disagrees with the adjustments proposed by the IRS and intends to defend its position. The timing of the resolution of these issues is uncertain, however, it is reasonably possible that the resolution could occur within the next twelve months. An estimate of the range of potential outcomes cannot be made at this time. When ultimately settled, White Mountains does not expect the resolution of this examination to result in a material change to its financial position. 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. White Mountains does not expect the resolution of this examination to result in a material change to its financial position. On December 15, 2011, the IRS commenced an examination of the income tax returns for 2010 for certain U.S. subsidiaries of AFI. Pursuant to a Stock Purchase Agreement dated as of May 17, 2011 between White Mountains and Allstate, White Mountains is required to indemnify Allstate for any changes in pre-closing taxes. White Mountains does not expect the resolution of this examination to result in a material change to its financial position. The IRS conducted an examination of income tax returns for 2006 and 2007 for certain U.S. subsidiaries of Sirius Group. On October 26, 2011, the Sirius Group received and signed the IRS Revenue Agent’s Report, which contained no proposed adjustments. The IRS also examined the U.S. income tax return filed by WM Belvaux S.à r.l., a Luxembourg subsidiary, for tax year 2007. On May 3, 2011, the exam was completed with no proposed adjustments. The Ministry of Finance in Sweden has proposed reducing the corporate income tax rate in Sweden to 22.0% from 26.3%. If this proposal is enacted, the reduction in the Swedish corporate income tax rate would reduce the White Mountains' net deferred tax liability by approximately $60 million. The Ministry of Finance in Sweden has also proposed tax legislation that, if enacted, would limit the deductibility of interest paid on certain intra-group debt instruments after January 1, 2013. At September 30, 2012, White Mountains has $272.1 million of deferred tax assets related to net operating loss carryforwards that are being utilized from interest income on intra-group debt instruments that would be affected by the proposed tax legislation. However, management does not expect that the proposed tax legislation will adversely affect the carrying value of these deferred tax assets as management believes it has alternate strategies to utilize the deferred tax assets.

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 Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Luxembourg, the Netherlands, Singapore, Sweden, Switzerland, the United Kingdom and the United States.

White Mountains' income tax expense for the three months ended September 30, 2012 represented an effective tax rate of 35.4%, which is in line with the U.S. statutory rate of 35%. White Mountains’ income tax expense for the nine months ended September 30, 2012 represented effective tax rate of 27.2%, which differed from the U.S. statutory rate of 35% primarily due to income generated in jurisdictions other than the United States.

White Mountains' effective tax rate for the three and nine months ended September 30, 2011 was not meaningful as pre-tax income was near break-even.

In arriving at the effective tax rate for the three and nine months ended September 30, 2012 and 2011, White Mountains forecasted the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2012 and 2011 and included these gains (losses) in the effective tax rate calculation pursuant to ASC 740-270.

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 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. During the next twelve months, it is possible that certain planning strategies will no longer be sufficient to utilize the entire deferred tax asset, which could result in material changes to White Mountains’ deferred tax assets and tax expense.

Upon completion of the Runoff Transaction, it is expected that the unrecognized tax benefits associated with tax positions where the deductibility is certain but the timing is uncertain, will decrease by approximately $11.5 million. White Mountains does not expect the decrease to result in a material change to its financial position.

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 2005.

The IRS is conducting an examination of income tax returns for 2005 and 2006 for certain U.S. subsidiaries of OneBeacon. On January 5, 2011, White Mountains received Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2005 and 2006. The estimated total assessment, including interest and utilization of alternative minimum and foreign tax credit carryovers, is $20.6 million. White Mountains disagrees with the adjustments proposed by the IRS and intends to defend its position. The timing of the resolution of these issues is uncertain, however, it is reasonably possible that the resolution could occur within the next twelve months. An estimate of the range of potential outcomes cannot be made at this time. When ultimately settled, White Mountains does not expect the resolution of this examination to result in a material change to its financial position.

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. White Mountains does not expect the resolution of this examination to result in a material change to its financial position.

On December 15, 2011, the IRS commenced an examination of the income tax returns for 2010 for certain U.S. subsidiaries of AFI. Pursuant to a Stock Purchase Agreement dated as of May 17, 2011 between White Mountains and Allstate, White Mountains is required to indemnify Allstate for any changes in pre-closing taxes. White Mountains does not expect the resolution of this examination to result in a material change to its financial position.

The IRS conducted an examination of income tax returns for 2006 and 2007 for certain U.S. subsidiaries of Sirius Group. On October 26, 2011, the Sirius Group received and signed the IRS Revenue Agent’s Report, which contained no proposed adjustments. The IRS also examined the U.S. income tax return filed by WM Belvaux S.à r.l., a Luxembourg subsidiary, for tax year 2007. On May 3, 2011, the exam was completed with no proposed adjustments.

The Ministry of Finance in Sweden has proposed reducing the corporate income tax rate in Sweden to 22.0% from 26.3%. If this proposal is enacted, the reduction in the Swedish corporate income tax rate would reduce the White Mountains' net deferred tax liability by approximately $60 million. The Ministry of Finance in Sweden has also proposed tax legislation that, if enacted, would limit the deductibility of interest paid on certain intra-group debt instruments after January 1, 2013. At September 30, 2012, White Mountains has $272.1 million of deferred tax assets related to net operating loss carryforwards that are being utilized from interest income on intra-group debt instruments that would be affected by the proposed tax legislation. However, management does not expect that the proposed tax legislation will adversely affect the carrying value of these deferred tax assets as management believes it has alternate strategies to utilize the deferred tax assets.