Income Taxes	3 Months Ended
Mar. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes OneBeacon and its Bermuda-domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event that there is a change in the current law such that taxes are imposed, OneBeacon 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. OneBeacon also has subsidiaries that operate in Gibraltar, Luxembourg and the United States. U.S. operations are financed with a combination of debt and equity and the financing income currently accounts for the majority of non-U.S. earnings. OneBeacon's income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2014 and 2013, represented net effective tax rates of 11.9% and 22.5% respectively. The effective tax rate for the three months ended March 31, 2014, was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate and the settlement of the 2005-2006 IRS exam. The effective tax rate for the three months ended March 31, 2013, was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate. For the three months ended March 31, 2014 and 2013, the effective tax rate on non-U.S. income was 1.3% and (0.1)%, respectively, and the effective tax rate on U.S. income was 16.0% and 29.4%, respectively. In arriving at the effective tax rate for the three months months ended March 31, 2014 and 2013, OneBeacon forecasted all income and expense items including the realized and change in unrealized investment gains for the years ending December 31, 2014 and 2013, and included these gains in the effective tax rate calculation. OneBeacon classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. With few exceptions, OneBeacon is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2005. On February 14, 2014, OneBeacon received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Over assessment) from the IRS Appeals Office relating to the examination of tax years 2005 and 2006. All disputed items have now been agreed and resolved with the Joint Committee. OneBeacon recorded a tax benefit of $5.0 million in the first quarter of 2014 relating to the settlement of the IRS examination for tax years 2005 and 2006. On July 28, 2011, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2007, 2008 and 2009. On July 17, 2013, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. The estimated total assessment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $69.3 million. However, $60.2 million of the proposed adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the deduction in the exam period would not affect the effective tax rate but would accelerate the payment of cash to the taxing authority. OneBeacon disagrees with the adjustments proposed by the IRS and is defending its position. 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. An estimate of the range of potential outcomes cannot be made at this time. OneBeacon does not expect the resolution of this examination to result in a material change to its financial position. On September 2, 2013, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2010, 2011 and 2012. OneBeacon does not expect the resolution of this examination to result in a material change to its financial position.

Income Taxes

OneBeacon and its Bermuda-domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event that there is a change in the current law such that taxes are imposed, OneBeacon 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. OneBeacon also has subsidiaries that operate in Gibraltar, Luxembourg and the United States. U.S. operations are financed with a combination of debt and equity and the financing income currently accounts for the majority of non-U.S. earnings.

OneBeacon's income tax expense related to pre-tax income from continuing operations for the three months ended March 31, 2014 and 2013, represented net effective tax rates of 11.9% and 22.5% respectively. The effective tax rate for the three months ended March 31, 2014, was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate and the settlement of the 2005-2006 IRS exam. The effective tax rate for the three months ended March 31, 2013, was lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate. For the three months ended March 31, 2014 and 2013, the effective tax rate on non-U.S. income was 1.3% and (0.1)%, respectively, and the effective tax rate on U.S. income was 16.0% and 29.4%, respectively.

In arriving at the effective tax rate for the three months months ended March 31, 2014 and 2013, OneBeacon forecasted all income and expense items including the realized and change in unrealized investment gains for the years ending December 31, 2014 and 2013, and included these gains in the effective tax rate calculation.

OneBeacon classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. With few exceptions, OneBeacon is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2005. On February 14, 2014, OneBeacon received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Over assessment) from the IRS Appeals Office relating to the examination of tax years 2005 and 2006. All disputed items have now been agreed and resolved with the Joint Committee. OneBeacon recorded a tax benefit of $5.0 million in the first quarter of 2014 relating to the settlement of the IRS examination for tax years 2005 and 2006.

On July 28, 2011, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2007, 2008 and 2009. On July 17, 2013, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. The estimated total assessment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $69.3 million. However, $60.2 million of the proposed adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the deduction in the exam period would not affect the effective tax rate but would accelerate the payment of cash to the taxing authority. OneBeacon disagrees with the adjustments proposed by the IRS and is defending its position. 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. An estimate of the range of potential outcomes cannot be made at this time. OneBeacon does not expect the resolution of this examination to result in a material change to its financial position.

On September 2, 2013, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2010, 2011 and 2012. OneBeacon does not expect the resolution of this examination to result in a material change to its financial position.