Nature of Operations and Summary of Significant Accounting Policies Acquisitions and Dispositions Description of various transactions that are significant to the Company's financial position, results of operations and/or cash flows. Significant Transactions [Text Block] Variable Interest Entities Disclosure [Abstract] Variable Interest Entities [Text Block] Represents the entire disclosure of variable interest entities (VIE), including, but not limited to the nature, purpose, size, and activities of the VIE, the carrying amount and classification of consolidated assets that are collateral for the VIE's obligations, lack of recourse if creditors (or beneficial interest holders) of a consolidated VIE have no recourse to the general credit of the primary beneficiary. An enterprise that holds a significant variable interest in a VIE but is not the primary beneficiary may disclose the nature of its involvement with the VIE and when that involvement began, the nature, purpose, size, and activities of the VIE and the enterprise's maximum exposure to loss as a result of its involvement with the VIE. Variable Interest Entities Document And Entity Information [Abstract] Statutory Capital And Surplus Disclosure [Abstract] This element represents Statutory Capital and Surplus. Statutory Capital and Surplus Disclosure [Text Block] Statutory Capital and Surplus This element represents the entire disclosure for Statutory Capital and Surplus. Share Repurchase Program Represents the entire disclosure pertaining to the company's share repurchase program. Share Repurchase Program Disclosure [Text Block] Share Repurchase Program Disclosure [Abstract] Share Repurchase Program This element represents Share Repurchase Program. Investment Sector [Axis] Payments for Distribution to Parent Entity Distribution to White Mountains Insurance Group, Ltd. Represents the amount paid to the parent company due to increase in income taxes in final determination of federal income tax examination. Investment Sector [Domain] Amendment Description Distributions to Parent Entity Distribution to White Mountains Insurance Group, Ltd. Represents the amount of distributions made to the parent entity. Amendment Flag Other Comprehensive Income Other Adjustments Net Of Tax Period Increase Decrease Change in other comprehensive income and loss items This element represents Other Comprehensive Income or Loss, Net of Tax, for the period. Includes deferred gains or losses on qualifying hedges, minimum pension liability, and cumulative translation adjustment. Adjustments to Additional Paid in Capital Gain on Sale of Business Increase in additional paid in capital due to gain on sale of business unit during the reporting period. Gain on sale of OneBeacon Holdings (Luxembourg) S.a r.l. Gain on sale of OneBeacon Holdings (Luxembourg) S.a r.l. Discontinued Assets Represents assets held for sale Discontinued Assets Policy Acquisition Costs Pronouncement Policy [Text Block] Disclosure of impact from recent accounting pronouncement (ASU 2010-26) on accounting for policy acquisition costs. Policy Acquisition Costs Comprehensive Net Income Policy [Text Block] Comprehensive Income Represents the accounting policy of the entity with respect to comprehensive net income of the entity. White Mountains Insurance Group, Ltd. Represents the White Mountains Insurance Group, Ltd. White Mountains Insurance Group Ltd [Member] Disclosure of accounting policy for impairment of goodwill. Goodwill Impairment Impairment of Goodwill Policy [Text Block] Schedule of One Beacon Top Reinsurers [Table Text Block] Tabular disclosure of the top five reinsurers that have assumed business from OneBeacon (excluding industry pools and associations) based on the amount recoverable from each reinsurer. The disclosure for each reinsurer also includes the amount recoverable as a percentage of OneBeacon's consolidated reinsurance recoverables and the A.M Best Company, Inc. ("A.M. Best") rating. Listing of top reinsurers Schedule of Selected Balance Sheet Data by Segment [Table Text Block] Tabular disclosure of selected balance sheet data for each reportable segment. Schedule of selected balance sheet of the segments Schedule of selected underwriting results Tabular disclosure of selected underwriting results for each reportable segment. Schedule of Selected Underwriting Results Schedule of Nonvested Performance Share Activity [Table Text Block] Tabular disclosure of the changes in nonvested performance shares outstanding and the amount accrued for performance shares. Summary of performance share activity Schedule of Performance Shares Outstanding and Accrued Expense by Performance Cycle Underincentive Plan [Table Text Block] Tabular disclosure of performance shares outstanding and the amount accrued by performance cycle, under the entity's Incentive Plan. Summary of performance shares outstanding and accrued expense for performance shares awarded under the OB Incentive Plan Schedule of Share Based Compensation One Beacon Stock Options Activity [Table Text Block] Disclosure of changes in the number of target options outstanding and the amount accrued for options. Summary of option activity Schedule of Nonvested One Beacon Restricted Share Activity [Table Text Block] Tabular disclosure of the changes in nonvested restricted shares and the unamortized grant date fair value. Summary of restricted shares activity Schedule of discontinued operations balance sheet Schedule of Discontinued Operations Balance Sheet [Table Text Block] Tabular disclosure of the classification and carrying value of the assets and liabilities of discontinued operations. Schedule of Discontinued Operations Income Statement [Table Text Block] Tabular disclosure of gain (loss) recognized in the income statement and the income statement caption that includes that gain (loss), amounts of revenues and pretax profit or loss reported in discontinued operations. Schedule of discontinued operations income statement Current Fiscal Year End Date Schedule of Discontinued Operations Calculation of Numerator and Denominator in Earnings Per Share [Table Text Block] Tabular disclosure of the numerators and the denominators of the basic and diluted per-share (or per-unit) computations for income from discontinued operations, including the effect that has been given to preferred dividends. Discontinued operations, computation of earnings per share Trading Securities and Certain Trading Assets Fixed Maturity Investments [Table Text Block] Tabular disclosure of fixed maturity securities for which an entity has elected the fair value option. Investment holdings, fixed maturity investments Investment Fixed Maturity Securities Credit Quality Indicators [Table Text Block] Tabular disclosure of fixed maturity securities by credit quality indicator. Debt securities, credit ratings Fair Value Assets Measured on Recurring Basis Measurement Input Reconciliation [Table Text Block] Tabular disclosure of the fair value measurement of assets measured at fair value on a recurring basis, including a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: total gains or losses for the period (realized and unrealized), amortization or accretion of discount, purchases, sales, issuances, transfers in and transfers out of each level of the fair value hierarchy by class of asset. Rollforward of fair value measurements by level Unrealized Gain (Loss) on Investments Unobservable Inputs [Table Text Block] Tabular disclosure of the amount of total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments. Net unrealized gains or losses for Level 3 investments Accretion (Amortization) of Discounts and Premiums, Investments Amortization/accretion Schedule of fair Value Assets Measured on Recurring Basis Mortgage and Asset Backed Securities [Table Text Block] Tabular disclosure of mortgage-backed securities and other asset-backed securities that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). Mortgage-backed and asset-backed securities Schedule of Security Issuance Years of Investments in Mortgage Backed Securities Issued by Private Enterprises [Table Text Block] Tabular disclosure of security issuance years of investments in non-agency RMBS and non-agency CMBS securities. Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities Non Agency Residential Mortgage Backed Securities Credit Quality Indicators and Seniority [Table Text Block] Tabular disclosure of non-agency residential mortgage-backed securities by credit quality indicator and seniority. Non-agency RMBS, collateral quality and tranche levels Non Agency Commercial Mortgage Backed Securities Interest Rate and Seniority [Table Text Block] Tabular disclosure of non-agency commercial mortgage-backed securities by type and by seniority level. Non-agency CMBS, type of interest rate and tranche levels Hedge Fund and Private Equity Investment by Objective or Sector [Table Text Block] Tabular disclosure of hedge funds and private equity investments held at fair value by investment objective or sector and the amount of unfunded commitments associated with each objective or sector. Other investments Hedge Fund Redemption Frequency Notice Period [Table Text Block] Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds. Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds Document Period End Date Private Equity Lock Up Period [Table Text Block] Tabular disclosure of expected lock up period of private equity investments. Fair Value of private equity funds subject to lock-up periods Fair Value Inputs Assets Quantitative Information [Table Text Block] Tabular disclosure of quantitative information about the inputs used in the fair value measurement of assets. This disclosure may include, but is not limited to, the fair value of the asset, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs. Schedule of significant unobservable inputs used in estimating the fair value of investment securities Ceded Share Auto One Written Percentage The percentage of the AutoOne business written by the entity which is ceded on a quota share basis. Ceded share of AutoOne business (as a percent) Ceded Share Runoff Business Written Percentage Ceded share of Runoff Business (as a percent) The percentage of Runoff Business written by the entity, which is ceded on a quota share basis. Ceded Share Ongoing Business Written Percentage Ceded share of ongoing business (as a percent) The percentage of ongoing business written by the entity, which is ceded on a quota share basis. Represents AutoOne which provides products and services to automobile assigned risk markets. AutoOne Auto One [Member] AutoOne Transaction Run off Business [Member] Runoff Business Represents information pertaining to Runoff Business of the entity. Runoff Atlantic Specialty Insurance Company [Member] ASIC Represents Atlantic Specialty Insurance Company. Policyholder Benefits and Claims Incurred Net Discontinued Operations Provision for benefits, claims and claims settlement expenses incurred during the period net of the effects of contracts assumed and ceded related to discontinued operations. Total incurred loss and LAE from discontinued operations Entity [Domain] Liability for Unpaid Claims and Claims Adjustment Expense Claims Paid Discontinued Operations Total amount of payments made during the reporting period to settle insured claims and pay costs incurred in the claims settlement process related to discontinued operations. Total loss and LAE payments from discontinued operations Loss and Loss Adjustment Expense Reserves Reclassified to from Held for Sale Net loss and LAE reserves reclassified to held for sale Represents the amount of loss and loss adjustment expense reserves reclassified (to) from held for sale. Loss and Loss Adjustment Expense Reserves Sold Net loss and LAE reserves sold Represents the amount of loss and loss adjustment expense reserves sold. Loss And Loss Adjustment Expense Reserves [Table] Schedule of loss and loss adjustment expense ("LAE") reserve activities. Represents the segment of the entity that offers specialty lines. Specialty Insurance Operations Segment [Member] Represents the segment of the entity that includes other insurance operations. Other Insurance Operations Segment [Member] Loss And Loss Adjustment Expense Reserves [Line Items] Reserves for Unpaid Losses and Loss Adjustment Expenses Reinsurance [Line Items] Reinsurance Reinsurance related to balances classified as held for sale Property Catastrophe Reinsurance Program First Layer First layer of reinsurance agreement that is retained by the entity. Amount of loss on a single catastrophe retained, first layer Property Catastrophe Reinsurance Program Second Layer Second layer of reinsurance agreement, in excess of the first layer, that is reinsured. Amount of loss on a single catastrophe reinsured, second layer Property Catastrophe Reinsurance Program Second Layer First Retention Percentage Represents the retention percentage between $25 million and $40 million for the second layer of the reinsurance agreement. First retention percentage of loss on a single catastrophe reinsured, second layer (as a percent) Property Catastrophe Reinsurance Program First Retention Slab for Second Layer Represents the retention for the second layer of the reinsurance agreement. Amount of loss on a single catastrophe reinsured, first retention slab for second layer Property Catastrophe Reinsurance Program Second Layer Second Retention Percentage Represents the retention percentage between $40 million and $80 million for the second layer of the reinsurance agreement. Second retention percentage of loss on a single catastrophe reinsured, second layer (as a percent) Property Catastrophe Reinsurance Program Second Retention Slab for Second Layer The low end of the range of the retention for the second layer of the reinsurance agreement. Amount of loss on a single catastrophe reinsured, second retention slab for second layer Property Catastrophe Reinsurance Program Third Layer Third layer of reinsurance agreement, in excess of the second layer, that is retained by the entity. Amount of loss on a single catastrophe retained, third layer Reinsurance [Table] Schedule reflecting the name of each reinsurer from whom amounts are due and other details like percentage of amounts recoverable from each reinsurer and their respective ratings. Economic Entity [Axis] Economic entities which constitute neither defined legal entities nor reportable segments of the reporting entity. Economic Entity [Domain] A listing of economic entities which are not legal entities or defined reportable segments of the entity. National Indemnity Company And General Reinsurance Corporation [Member] Represents National Indemnity Company and General Reinsurance Corporation, which are third party reinsurers of the entity. National Indemnity Company and General Reinsurance Corporation The percentage ceded under quota share reinsurance agreement (as a percent) Percentage of Quota Share Reinsurance Agreement Represents the percentage ceded under quota share reinsurance agreement. Reinsurance Recoverables Gross Purchase Accounting Adjustments Purchase accounting adjustments related to reinsurance recoverables. Purchase accounting adjustments Percentage of Total Reinsurance Recoverables The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables. Percentage of total paid and unpaid reinsurance recoverables (as a percent) National Indemnity Company [Member] Represents National Indemnity Company, which is a third party reinsurer of the entity. National Indemnity Company General Reinsurance Corporation [Member] Represents General Reinsurance Corporation, which is a third party reinsurer of the entity. General Reinsurance Corporation Tower Insurance Company [Member] Represents Tower Insurance Company, which is a third party reinsurer of the entity. Tower Insurance Company Hannover Ruckversich [Member] Hannover Ruckversich Represents Hannover Ruckversich, which is a third party reinsurer of the entity. Hartford Steam Boiler [Member] Hartford Steam Boiler Represents Hartford Steam Boiler, which is a third party reinsurer of the entity. Transatlantic Reinsurance [Member] Transatlantic Reinsurance Represents Transatlantic Reinsurance, which is a third party reinsurer of the entity. Platinum Underwriters Re [Member] Platinum Underwriters Re Represents Platinum Underwriters Re, which is a third party reinsurer of the entity. Accrued Investment Income Receivable Investment income accrued Tokio Marine and Nichido Fire [Member] Represents Tokio Marine and Nichido Fire, which are third party reinsurers of the entity. Tokio Marine and Nichido Fire Munich Reinsurance America [Member] Represents Munich Reinsurance America, which is a third party reinsurer of the entity. Munich Reinsurance America Other Third Party Reinsurance [Member] Represents the various other third party reinsurers of the entity. Other Third Party Reinsurance Reinsurance Contract By Nature [Axis] Information by name that identifies a reinsurance contract by type. Reinsurance Contract Name [Domain] The name that identifies a reinsurance contract by type. NICO Cover [Member] Represents the full risk-transfer cover from National Indemnity Company in old asbestos and environmental ("A&E") claims and certain other exposures. NICO Cover GRC Cover [Member] Represents the adverse loss reserve development cover from General Reinsurance Corporation. GRC Cover Northeast Homeowners Business [Member] Represents Northeast homeowners business of the entity. Northeast Homeowners Business Property Catastrophe Reinsurance Program [Member] Represents the reinsurance program that provides coverage of the periodical and commercial property business as well as certain acts of terrorism. Property Catastrophe Reinsurance Program Third Party Recoverables Reinsurer Would Pay If Uncollected From Third Party Reinsurer Amount of third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers. Third party recoverables which reinsurer would pay if they are unable to collect from third party reinsurers Reinsurance Recoverables Guaranteed By Third Party Reinsurer Reinsurance recoverables from various third party reinsurers that are guaranteed under the terms of a 100% quota share reinsurance agreement. Reinsurance recoverable from various third party reinsurers, guaranteed by the third party reinsurer under reinsurance agreement Reinsurance Recoverable Maximum Cover Under Reinsurance Contract Represents the maximum cover amount under reinsurance contracts. Maximum cover under reinsurance contract Accumulated Other Comprehensive Income (Loss), Net of Tax Other comprehensive income and loss items Utilized Coverage From Reinsurer Represents the utilized coverage from reinsurer. Utilized coverage from NICO Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Accumulated other comprehensive loss, after tax: Reinsurance Receivables Paid By Reinsurer Represents amount of incurred losses paid by the reinsurer through the reporting date. Reinsurance receivables paid by the reinsurer Represents the percentage of utilized coverage from reinsurer that relates to uncollectible Third Party Recoverables. Percentage of utilized coverage from NICO that relates to uncollectible Third Party Recoverables (as a percent) Utilized Coverage Relating To Uncollectible Third Party Recoverables Percentage Utilized Coverage Relating to Uncollectible Third Party Recoverables Represents the utilized coverage from reinsurer that relates to uncollectible Third Party Recoverables. Utilized coverage from NICO that relates to uncollectible Third Party Recoverables Estimated Incurred Losses Ceded Represents the estimated amount of loss ceded as of the balance sheet date. Ceded estimated incurred losses Entity Well-known Seasoned Issuer Adverse Loss Reserve Development On Losses Occurring In Years 2000 And Prior Represents the amount of adverse loss reserve development on losses occurring in years 2000 and prior. Adverse loss reserve development on losses occurring in years 2000 and prior Entity Voluntary Filers Loss Reserves Ceded At Acquisition Represents the amount of reserves ceded as of the date of the OneBeacon acquisition. Reserves ceded as of the date of the OneBeacon acquisition Entity Current Reporting Status AM Best Rating System Number Of Ratings The number of ratings in the A.M. Best rating system. Number of ratings in A.M. Best ratings Entity Filer Category Number Of Contracts Seller Caused OneBeacon To Purchase Prior To Acquisition Represents the number of reinsurance contracts the seller caused OneBeacon to purchase from subsidiaries of Berkshire Hathaway Inc. immediately prior to the entity's acquisition of OneBeacon. Number of reinsurance contracts the seller caused OneBeacon to purchase immediately prior to the entity's acquisition of OneBeacon Entity Public Float Onebeacon U S Holdings Inc [Member] Represents OneBeacon U.S. Holdings, Inc. OneBeacon U.S. Holdings, Inc. ("OBH") Entity Registrant Name Onebeacon Insurance Company [Member] Represents the OneBeacon Insurance Company. OBIC Entity Central Index Key Extinguishment of Debt [Domain] The group of events related to debt extinguishment. Extinguishment Of Debt One [Member] The tender offer on June 1, 2010. Tender Offer Extinguishment Of Debt Two [Member] The repurchase and retirement of debt in addition to the tender offer which occurred in the second quarter of 2010. Additional repurchase and retirement of debt Extinguishment Of Debt Three [Member] The repurchase and retirement of debt which occurred in the first quarter of 2010. Repurchase and retirement of debt Entity Common Stock, Shares Outstanding Debt Instrument Issue Price As Percentage Of Par Value Represents the price of debt issued as a percentage of its par value. Percentage of par value at which debt was issued Deferred Underwriting Discount Gross Represents the amount of capitalized underwriting costs associated with the issuance of debt instruments that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization. Underwriting discount Investing Financing And Corporate Operations Segment [Member] Represents the segment of the entity that includes investing, financing and corporate operations. Pretax Income Loss Sum of operating profit and nonoperating income or expense before income taxes. Pre-tax income (loss) Major Underwriting Unit Represents the number of major underwriting units for financial reporting. Major underwriting unit Statement, Major Underwriting Units [Axis] The major underwriting units reported in a statement. Major Underwriting Unit [Domain] Major underwriting unit representing facts about an entire consolidated business entity disaggregated by business or economic activities. MGA Business [Member] Major underwriting unit comprised of collector cars and boats, Dewar and OBE. MGA Business Specialty Industries [Member] Major underwriting unit comprised of IMU, OBTI, OBA, OBGR and OBEG. Specialty Industries Specialty Products [Member] Major underwriting unit comprised of OBPI, PIM, OBSP and OBES. Specialty Products Loss and LAE Ratio The ratio of incurred loss and LAE to earned premiums Loss and LAE Ratio (as a percent) Expense Ratio The ratio of policy acquisition and other underwriting expenses to earned premiums. Expense Ratio (as a percent) Combined Ratio The ratio calculated by adding the loss and LAE ratio and the expense ratio. Total GAAP combined Ratio (as a percent) Total Underwriting Income Loss The difference between earned premiums and the sum of loss and LAE, policy acquisition expenses and other underwriting expenses. Total underwriting income (loss) Defined Benefit Plan Before Settlements Curtailments And Special Termination Benefits The total amount of periodic benefit cost before settlements, curtailments and special termination benefits for defined benefit plans for the period. Net periodic pension (income) cost before settlement and special termination benefits expense Defined Benefit Plan Employer Contributions in Current Fiscal Year The employer's best estimate, as soon as it can be reasonably determined, of contributions paid to the plan in the current fiscal period. Contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. Contributions to the Non-qualified Plan in the current fiscal period Share Based Compensation Arrangementby Share Based Payment Award Equity Instruments Other than Options Target Shares Paid in Period The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. Payments and deferrals (in shares) Document Fiscal Year Focus Accrued Expense Rollforward [Abstract] Expense recognized Document Fiscal Period Focus Accrued expense at beginning of period Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. Accrued expense at end of period Accrued incentive compensation Incentive Compensation Payable Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Amounts Paid in Period The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. Payments and deferrals Employee Service Share Based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not Yet Recognized The total reduction to unamortized compensation cost for restricted shares forfeited during the period. Forfeitures and net change in assumed forfeitures This element represents percentage of target achieved by employee under performance shares during the reporting period. Percentage of target achieved (as a percent) Percentage of Target Achieved Target performance shares outstanding, beginning of period Target performance shares outstanding, end of period Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures This element represents the number of non-vested performance shares before forfeiture assumptions. Incentive Compensation Payable before Forfeitures Carrying value as of the balance sheet date for performance shares before forfeitures. Accrued expense ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresInPeriodAmount This element represents the reduction to accrued expense for assumed forfeitures of performance shares. Assumed forfeitures Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Assumed Forfeitures in Period This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. Assumed forfeitures (in shares) Performance Share Target Vesting Percentage This element represents the target percentage vesting for the performance shares. Target vesting percentage of performance shares (as a percent) Share Based Compensation Arrangement by Share Based Payment Award, Additional Cost to be Recognized if Performance Shares Vested This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested. Additional compensation cost that would be recognized if all outstanding performance shares vested This element represents OneBeacon stock options that vested as a result of the Commercial Lines Transaction and the Personal Lines Transaction. The OneBeacon options were not exercised (as they were out-of-the-money) and subsequently expired. Vested and expired (options) Share Based Compensation Arrangement by Share Based Payment Award, Options Vested and Expirations in Period Legal Entity [Axis] Share Based Compensation Arrangement by Share Based Payment Award, Fair Value Assumptions Forfeiture The rate of forfeitures that is used in valuing an option. Forfeiture (as a percent) Document Type Initial exercise price (in dollars per share) Share Based Compensation Arrangement by Share Based Payment Award, Options Initial Exercise Price This element represents the initial exercise price per option to purchase common shares. Share Based Compensation Arrangement by Share Based Payment Award, Options Outstanding One Beaconamended Exerciseprice This element represents the amended exercise price per OneBeacon non-qualified option. The exercise price was amended as a result of the special dividend(s) paid on OneBeacon common shares. Amended exercise price of option (in dollars per share) Special Dividend Per Share Special dividend per common share paid by One Beacon. OneBeacon special dividend (in dollars per share) Proportion of Other than Option Vesting Tranche in Each Year This element represents the proportion of RSUs that vest on each of November 2009, 2010 and 2011. Proportion of award vests on each of Nov 9, 2009, 2010 and 2011 (as a percent) Percentage of Growth in Book Value Per Share Condition for Vesting This element represents the minimum percentage of growth in book value per share from January 1, 2008 through the end of the calendar year immediately following the applicable vesting date required to trigger the vesting of restricted stock units. Minimum percentage of growth in book value per share for vesting each tranche of units (as a percent) Share Based Compensation Arrangement by Share Based Payment Award, Equity Instruments Other than Options Reduction in Grant Total Represents the total number of performance share grants that will be reduced. Total number of performance share grants to be reduced (in shares) Share Based Compensation Arrangement by Share Based Payment Award, Equity Instruments Other than Options Expected to Vest Numerator Represents the numerator value of the ratio in which shares will vest. Numerator value of the ratio in which restricted shares vest (as a percent) Share Based Compensation Arrangement by Share Based Payment Award, Equity Instruments Other than Options Expected to Vest Denominator Represents the denominator value of the ratio in which shares will vest. Denominator value of the ratio in which restricted shares vest (as a percent) Years Performance Shares Will be Forfeited Represents the number of years that the CEO of a subsidiary will forfeit performance shares of that subsidiary. Period in which performance share awards will be reduced by a similar number of shares (in years) Pertinent data describing and reflecting required disclosures pertaining to an equity-based compensation arrangement, by plan names. Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Plan Name [Axis] Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Plan Name [Domain] Types of equity-based compensation plans of the entity. One Beacon Long Term Incentive Plan The share-based incentive plans of the entity. One Beacon Long Term Incentive Plan [Member] Schedule of Share Based Compensation Arrangement By Share Based Payment Award Award Type [Axis] Pertinent data describing and reflecting required disclosures pertaining to an equity-based compensation arrangement, by awards name. Schedule Of Share Based Compensation Arrangement By Share Based Payment Award Award Type [Domain] Types of equity-based compensation awards of the entity. Performance Share Represents conditional grants of a specified number of common shares or an equivalent amount of cash. Performance Share [Member] Nonqualified Stock Options [Member] This element represents stock options which do not qualify for the special tax treatment accorded to incentive stock options. Nonqualified Stock Options Share Based Compensation Arrangement By Performance Shares Award Performance Cycle [Axis] Disclosure of minimum performance period for right to receive performance share from the entity. This element represents minimum performance period for right to receive performance share from the entity. Share Based Compensation Arrangement By Performance Shares Award Performance Cycle [Domain] Range Of Performance Cycle From 2009 To 2011 [Member] This element represents period of performance cycle from 2009 to 2011. Performance cycle 2009-2011 Performance cycle 2010-2012 Range Of Performance Cycle From 2010 To 2012 [Member] This element represents period of performance cycle from 2010 to 2012. Range Of Performance Cycle From 2011 To 2013 [Member] This element represents period of performance cycle from 2011 to 2013. Performance cycle 2011-2013 Range Of Performance Cycle From 2012 To 2014 [Member] This element represents period of performance cycle from 2012 to 2014. Performance cycle 2012-2014 One Beacon Restricted Stock [Member] Represents restricted shares as awarded by the company to their employees as a form of incentive compensation. Restricted stock are shares of stock for which sale is contractually or governmentally restricted for a given period of time. Restricted Shares Trading Securities Unrealized Foreign Currency Transaction Gain (Loss) The unrealized foreign currency transaction gain (loss) as of the balance sheet date which is included in net income before tax during the reporting period. Changes in net foreign currency translation gains and losses Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. Total net changes in fair value reflected in revenues Trading Securities Amortized Cost This item represents the cost of trading securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. Cost or amortized cost Trading Securities Gross Unrealized Gain The gross amount of unrealized gains for trading securities as of the balance sheet date. Gross unrealized gains The gross amount of unrealized losses for trading securities as of the balance sheet date. Gross unrealized loss Trading Securities Gross Unrealized Loss Net foreign currency gains (losses) Trading Securities Net Unrealized Foreign Currency Gain (Loss) The net amount of unrealized gains (losses) from foreign currency transaction adjustments related to trading securities. Fair Value Assets Measured on Recurring Basis Gain (Loss) Included in Earnings This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. Total net realized and unrealized gains (losses) Fair Value Assets Measured on Recurring Basis Transfers In This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. Transfers in Fair Value Assets Measured on Recurring Basis Transfers Out This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. Transfers out Unrealized Gains (Losses) on Investments The aggregate increase (decrease) in the market value of unsold investments whose gains (losses) are included in earnings. Total The realized loss in the period related to the impairment of other investments. Realized impairment loss Realized Loss Impairment other Investments Change in Deferred Gains (Losses) Changes in deferred gains and losses on sales of investments between entities under common control. Change in deferred gains and losses Gross Unrealized Gains This item represents the gross unrealized gains for securities, at a point in time, which are categorized as trading securities Goss unrealized investment gains Gross Unrealized Investment Losses The gross unrealized losses for securities, at a point in time, which are categorized as trading securities Gross unrealized investment losses Gross Unrealized Gains (Losses) on Trading Securities Pretax The net unrealized gains (losses) for securities, at a point in time, which are categorized as trading securities Net unrealized gains from trading portfolio Income taxes Trading Securities Tax on Unrealized Holding Gains (Losses) The tax effect on the net unrealized gains losses for securities, at a point in time, which are categorized as trading securities. Income taxes on net unrealized investment gains The net unrealized gains (losses) for securities, after taxes, at a point in time, which are categorized as trading securities Total net unrealized investment gains, after tax Gross Unrealized Gains (Losses) on Trading Securities after Tax AutoOne assets reclassified to assets held for sale Auto One Fixed Maturities Investments AutoOne investments that are reclassified to assets held for sale on the balance sheet. Percentage of Fair Value Investments Using Observable Inputs The percentage of the investment portfolio recorded at fair value that was priced based upon observable inputs. Percentage of investments based on observable inputs This element represents the total fair market value of investments in certain entities that calculate net asset value per share (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements, but may also be used in both the balance sheet and disclosure in the same submission. This item represents alternative investments as of the balance sheet date which may include investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds, and any other investment in which investee management calculates net asset value per share as the primary metric for investment value. Other investments at fair value Other Long Term Investments Fair Value Disclosure Number of Limited Partnership and Limited Liability Corporation Investments Number of investments in limited partnership and limited liability corporations held as of the balance sheet date. Number of alternative investments Other Long Term Investments Largest Single Investment The largest investment which the entity has made in a single fund. Largest single investment in a fund Investment in Community Reinvestment Vehicle Represents an investment in a community reinvestment vehicle. Community reinvestment vehicle Tax Development Fund Investment Represents an investment in a tax advantaged federal affordable housing development fund. Tax development fund Investment in tax advantaged federal affordable housing development fund Foreign Government Excluding United States Debt Securities [Member] This category includes information about debt securities (such as bonds) issued by a national (excluding the United States), local, or municipal government not within the nation of domicile of the reporting entity. Foreign government obligations Consumer [Member] Represents the consumer sector. Consumer Industrial [Member] Represents the industrial sector. Industrial Financials [Member] Represents the financial sector. Financials Communications [Member] Represents the communications sector. Communications Energy [Member] Represents the energy sector. Energy Basic Materials [Member] Represents the basic materials sector Basic Materials Utilities [Member] Represents the utilities sector. Utilities Technology [Member] Represents the technology sector. Technology Other Sector [Member] Represents all other sectors that are not otherwise covered by the taxonomy. Other Sector Effective Income Tax Rate Continuing Operations Non US A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis non-U.S. pretax income from continuing operations. Effective tax rates non-U.S. (as a percent) A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis U.S. pretax income from continuing operations. Effective tax rates U.S. (as a percent) Effective Income Tax Rate Continuing Operations U S Private Preferred Stock Percentage The percentage of the preferred stock portfolio represented by one private preferred stock. Percentage of private preferred stock Schedule of Corporate Debt Security Credit Ratings [Axis] Credit ratings of corporate debt securities. Debt Credit Rating [Domain] External credit-ratings (for example, Standard & Poor's, Fitch, and Moody's) on corporate debt securities. Other Other Standard Poors Rating [Member] Remainder of ratings not separately disclosed. Includes information about OneBeacon segment consisting of Specialty Insurance Operations. Specialty Insurance Operations [Member] Specialty Insurance Operations Other Insurance Operations [Member] Includes information about OneBeacon segment consisting of Other Insurance Operations. Other Insurance Operations Investing Financing and Corporate Operations [Member] Includes information about OneBeacon segment Investing, Financing and Corporate Operations. Investing, Financing and Corporate Operations Income (Loss) from Continuing Operatings Excluding Portion Attributable to Noncontrolling Interest Represents Net income from continuing operations attributable to OneBeacon's common shareholders. Net income attributable to OneBeacon's common shareholders Income (Loss) Allocated to Participating Unvested Restricted Common Shares Represents the portion of income (loss) allocated to participating unvested restricted common shares. Allocation of income for participating restricted common shares Aggregate Allocation to Restricted Shares Including Dividend Represents the total amount allocated to restricted shares including dividend on restricted stock. Total allocation to restricted common shares Income Loss from Continuing Operatings Excluding Portion Attributable to Noncontrolling Interest Net Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. Net income attributable to OneBeacon's common shareholders, net of restricted share amounts Net income attributable to OneBeacon's common shareholders, net of restricted common share amounts Dividends Common Stock Cash Net of Restricted Common Share Amounts Equity impact of common stock cash dividends declared by the entity during the period, net of restricted share amounts. Dividends paid, net of restricted common shares Recategorized Investments Fair Value Disclosure Represents the securities recategorized from one fair value measurement level to another because quoted market prices for similar securities that were considered reliable could be validated against an alternative source that was available at the period end date. Transfers in and out of levels Fair Value Inputs Assets Quantitative Information [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axis the table. Fair Value Inputs Significant Unobservable Inputs Fair Value Inputs Assets Quantitative Information [Table] Schedule of the inputs used in the fair value measurement of assets. This disclosure may include, but is not limited to, the fair value of the asset, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs. Valuation Technique [Axis] Information by valuation technique. Valuation Technique [Domain] Valuation techniques used by the entity. Represents the discounted cash flow technique used to measure fair value. Discounted cash flow Discounted Cash Flow Valuation Technique [Member] Broker Pricing Cash Flow Valuation Technique [Member] Represents the broker pricing valuation technique used to measure fair value. Broker Pricing Cash Flow Valuation Technique Standard Poors NR Rating [Member] NR credit rating as defined by the external credit rating agency, Standard & Poor's. NR Represents the market convention for prepayment and default vector assumptions. Prepayment Rate (as a percent) Fair Value Inputs Prepayment and Default Rate Fair Value Inputs Discount Rate Over LIBOR Discount margin over LIBOR Interest rate over LIBOR, which is used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. Fair Value Inputs Discount Rate Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. Discount Yield (as a percent) Fair Value Inputs Loss Severity Represents the loss severity, used as an input to measure fair value. Loss Severity Fair Value Broker Quote Represents fair value of broker quote. Broker quote Payments of Dividends Common Stock Regular The cash outflow from the distribution of an entity's earnings in the form of regular dividends to common shareholders. Cash dividends declared and paid, regular The cash outflow from the distribution of an entity's earnings in the form of special dividends to common shareholders. Cash dividends declared and paid, special Payments of Dividends Common Stock Special Retained Share Autoone Business Written Percentage The percentage of the AutoOne business retained by the entity under the amendment of the pooling agreement. Retained Share of AutoOne business (as a percent) Average Basis Points of Subordination Average basis points of subordination Basis points of subordination on average. Commercial Mortgage Backed Securities Fixed Rate [Member] CMBS Fixed Rate Securities collateralized by commercial real estate mortgage loans which have a fixed interest rate. Commercial Mortgage Backed Securities Floating Rate [Member] CMBS Floating Rate Securities collateralized by commercial real estate mortgage loans which have a floating interest rate. Security Issuance Year [Axis] Represents the year of issuance for securities held by the entity. Represents the respective year in which the security was issued. Security Issuance Year [Domain] Securities Issued in 2007 [Member] Represents securities issued in 2007. 2007 Securities Issued in 2009 [Member] Represents securities issued in 2009. 2009 Securities Issued in 2010 [Member] 2010 Represents securities issued in 2010. Securities Issued in 2011 [Member] 2011 Represents securities issued in 2011. Securities Issued in 2012 [Member] 2012 Represents securities issued in 2012. Investment by Tranche Levels [Axis] Non agency RMBS by tranche Information by tranche level of non-agency RMBS securities and non-agency CMBS securities. Investment Tranche Levels [Domain] Tranche detail Tranche level indicators related to investments. Supersenior [Member] Super Senior Tranche This element represent the tranche level of Super Senior, which were rated AAA and were senior to other AAA bonds Senior [Member] Senior Tranche This element represent the tranche level of Senior, which were rated AAA and were senior to non-AAA bonds. Subordinate [Member] Subordinate Tranche This element represent the tranche level of Subordinate, which were not rated AAA and were junior to AAA bonds. Investment by Underlying Collateral Quality [Axis] Collateral Quality Information by the underlying collateral quality indicator. Investment Underlying Collateral Quality [Domain] Underlying collateral quality indicators related to investments. Collateral Quality Residential Nonprime Financing Receivable [Member] Class of financing receivables related to nonprime residential financing receivables. Residential Nonprime quality Non-prime Proceeds from Sale of Subsidiary The cash inflow associated with the amount received from the sale of a subsidiary during the period. Proceeds from sale of OneBeacon Holdings (Luxembourg) S.a r.l. Adjustments to Additional Paid in Capital Gain on Sale of Businessnet of Transaction Costs Increase in additional paid in capital due to gain on sale of business unit during the reporting period, net of transaction costs expensed through the statement of operations. Gain on sale of OneBeacon Holdings (Luxembourg) S.a r.l., net of transaction costs Investment Sector Detail [Axis] Represents an investments table in which investments are categorized by sector (industry, market, customer tier, etc.). Investment Sector Fair Value [Domain] Represents an investments table in which investments are categorized by sector (industry, market, customer tier, etc.). Promissory Note A written promise to pay a note to OneBeacon. Promissory note to OneBeacon Repayment of Promissory Note The cash inflow from a loan, supported by a promissory note. Repayment of Promissory Note to OneBeacon Prepayment of Principal on Promissory Note Amount of principal required to be pre-paid as a percentage of any dividend or distribution for Interboro received from its subsidiaries, net of taxes and less operating expenses. Prepayment of principal on promissory note (as a percent) Operating Expenses on Promissory Note Amount of operating expenses to be deducted from prepayment of principal upon receipt of dividend or distribution related to promissory note. Operating expenses related to promissory note Interest on Promissory Note Amount of interest expressed in basis points in addition to LIBOR rate. Interest on promissory note Distributions from Inactive Hedge Funds Distributions pending from hedge funds that are no longer active and are in process of disposing their underlying investments. Hedge funds in liquidation Fair Value Investments Entities that Calculate Net Asset Value Per Share Redemption Request The amount of redemption requests submitted for investments for hedge funds. Redemption Requests Hedge Funds and Private Equity Funds [Member] Investments in hedge funds and private equity funds. Hedge funds and private equity funds Hedge Funds Credit and Distressed [Member] Investments in registered hedge funds that invest in credit & distressed securities, taking both long and short position. Long/short credit and distressed Hedge Funds Diversified Strategies [Member] Investments in registered hedge funds that invest in diversified securities, taking both long and short positions. Diversified Strategies Hedge Funds Bank Loan [Member] Investments in registered hedge funds that invest in bank loans, taking both long and short positions. Long bank loan Hedge Funds Equity Activist [Member] Investments in registered hedge funds that invest in equity securities companies and use the ownership to be active in the management, taking both long and short positions. Long/short equity activist Represents investments in private equity funds of the insurance industry. Private Equity Insurance [Member] Insurance Private Equity Distressed Residential Real Estate [Member] Investments in private equity funds of distressed residential real estate. Distressed residential real estate Private Equity Energy Infrastructure and Services [Member] Investments in private equity funds related to energy infrastructure and services. Energy infrastructure and services Private Equity Healthcare [Member] Represents investments in private equity funds of the healthcare industry. Healthcare Investments in private equity funds of more than one industry. Private Equity Multi Sector [Member] Multi-sector Investments in private equity funds of the private equity secondary market. Private Equity Secondaries [Member] Private equity secondaries Investment Redemption Frequency [Axis] Information about the redemption frequency for investments, including (1) Monthly; (2) Quarterly; (3) Semi-annual; (4) Annual. Redemption frequency for investments, including (1) Monthly; (2) Quarterly; (3) Semi-annual; (4) Annual. Investment Redemption Frequency [Domain] Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Monthly [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment monthly. Monthly Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Quarterly [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment quarterly. Quarterly Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Annual [Member] For investments on which the entity calculates net asset value, the entity can redeem its investment annually. Annual Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency [Axis] The frequency with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency [Domain] The type of frequency with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated. For investments on which the entity calculates net asset value, the entity is required to deliver 30 to 59 days notice before it can redeem its investment. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period 30 to 59 Days [Member] 30 - 59 days notice Investment Redemption Notice Period 60 to 89 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver 60 to 89 days notice before it can redeem its investment. 60 - 89 days notice Investment Redemption Notice Period 90 to 119 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver 90 to 119 days notice before it can redeem its investment. 90 - 119 days notice Investment Redemption Notice Period Over 120 Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver over 120 days notice before it can redeem its investment. 120+ days notice Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period [Axis] The remaining period for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period [Domain] The identification of the remaining period for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. The period of from 1 to 3 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Liquidating Investment Remaining Period 1 to 3 Years [Member] 1 - 3 years The period of from 3 to 5 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. Liquidating Investment Remaining Period 3 to 5 Years [Member] 3 - 5 years Liquidating Investment Remaining Period 5 to 10 Years [Member] The period of from 5 to 10 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. 5 - 10 years Liquidating Investment Remaining Period over 10 Years [Member] The period greater than 10 years for those certain investments for which net asset value per share is calculated that the entity can never redeem, but from which the entity receives distributions through the liquidation of the underlying assets by the investee. More than 10 years Offsetting assets and liabilities Offsetting Assets And Liabilities Policy [Text Block] Represents the accounting policy of the entity with respect to offsetting assets and liabilities of the entity. Remaining Unamortized Fair Value Adjustment Represents the remaining unamortized amount of accreted fair value adjustment to loss and LAE reserves and reinsurance recoverable on unpaid losses related to purchase accounting. Remaining uanmortized fair value adjustment Third retention percentage of loss on a single catastrophe reinsured, second layer (as a percent) Property Catastrophe Reinsurance Program Third Layer Third Retention Percentage Represents the retention percentage between $80 million and $180 million for the second layer of the reinsurance agreement. Represents the third retention for the second layer of the reinsurance agreement. Amount of loss on a single catastrophe reinsured, third retention slab for second layer Property Catastrophe Reinsurance Program Third Retention Slab for Second Layer Hanover Insurance Company [Member] Represents Hanover Insurance Company, which is a third party reinsurer of the entity. Hanover Insurance Company Trading Securities, Gross Unrealized Gain (Loss) The net amount of gross unrealized gains (losses) for securities, at a point in time, which are categorized as trading securities. Gross unrealized investment gains or losses Fair value price adjustment level 3 security Represents the fair value adjustment made to the quoted market price based on internal procedures to validate the reasonableness of the price. Fair value adjustment of level 3 security Single Broker Indication [Member] Represents a single broker indication used to value a security. Single broker indication Price Cap Represents a cap put on the price of a private preferred stock. Price cap (as a percent) Performance cycle 2008-2010 Range Of Performance Cycle From 2008 To 2010 [Member] This element represents period of performance cycle from 2008 to 2010. Represents the number of insurance companies sold in a transaction. Number of insurance companies sold Number of Insurance Companies Sold Number of insurance entities sold Stock Issued during Period to Directors Number of shares issued during the period to directors. Shares issued to directors (in shares) Common Shares Issued [Abstract] Common Shares Repurchased And Retired [Abstract] Stock Repurchased and Retired Shares Number of shares that have been repurchased and retired under the share repurchase authorization. Common shares repurchased and retired (in shares) Employee Service Share Based Compensation Nonvested Awards Issued in Period Total Compensation Cost Not Yet Recognized The total fair value (on the date of issue) for restricted share awards issued during the period. Issued Cash Dividend Paid to Parent Company Equity impact of common stock cash dividends declared by an entity during the period. This element includes paid and unpaid dividends declared during the period. Dividends to OneBeacon U.S. Enterprises Holdings, Inc. Return of Capital This element represents Return of capital. Return of capital to OneBeacon U.S. Enterprises Holdings, Inc. Return of Capital Cash Outflow This element represents the cash payment on return of capital. Return of capital to OneBeacon U.S. Enterprises Holdings, Inc. Trading Securities Convertible Fixed Maturity Investments Convertible fixed maturity investments, at fair value Represents the fair value of convertible fixed maturity investments that are convertible to equity securities at a future date. Reinsurance recoverable on unpaid losses-Berkshire Hathaway, Inc. Reinsurance Recoverable on Unpaid Losses for Large Reinsurer The estimated amount recoverable as of the balance sheet date from Berkshire Hathaway, Inc. for claims incurred but not yet paid. Assets held for sale Assets held for sale Assets related to a disposition that have been classified as Held for Sale. Assets Held for sale Liabilites related to a disposition that have been classified as Held for Sale. Liabilities held for sale Liabilities Held for sale Other comprehensive income and loss items Accumulated Other Comprehensive Income (Loss) Net of Tax Other Includes the net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. Trading Securities Debt, Amortized Cost This element represents the cost of debt securities, which are categorized as trading, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any. Fixed maturity investments, amortized cost This element represents investments in common equity securities, categorized as trading, at cost. Trading Securities Equity, Cost Common equity securities, cost Trading Securities Convertible Bonds, Cost Convertible bonds, amortized cost This element represents the cost of convertible securities, which are categorized as trading, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any. Short-term investments, held in escrow Short Term Investments, Held in Escrow Represents the short term investments of the entity which are held in escrow relating to the net assets and liabilities under the quota share reinsurance agreements to be effected subsequently. Other investments, cost Other Investments Cost This element represents other investments not otherwise specified in the taxonomy at cost. Net Other Revenue Expense Net other revenues (expenses) Reflects the net of other revenues or gains and expenses or losses recognized by the entity in the period not otherwise specified in the income statement. Net other (expenses) revenues Net other revenues Policy acquisition expenses Insurance and Reinsurance Acquisition Expenses Amount of policy acquisition costs charged to expense in the period. Includes deferred policy acquisition costs amortized in the period, generally in proportion to related revenue earned over the policy or contract term. Policy acquisition expenses Accretion of Fair Value Adjustment to Loss And LAE Reserves Accretion of fair value adjustment to loss and LAE reserves Represents the part of the net reduction to loss and LAE reserves, which is accreted through a charge on the income statement rateably with and over the period the claims are settled. The reduction is caused as a result of the adjustment to the loss and LAE reserves to fair value on the balance sheet. The amount of gain (loss) resulting from the disposition of a discontinued operation during the period, net of related tax effect, per each basic and diluted share of common stock or unit when the per share amount is the same for both basic and diluted shares. Discontinued Operation Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Per Basic and Diluted, Share Loss from sale of discontinued operations, net of tax, per share (in dollars per share) Net loss from sale of discontinued operations Loss from sale of discontinued operations, net of tax After-tax gain (loss) resulting from the disposition of a discontinued operation. Gain (Loss) on Sale of Discontinued Operations Cash Reclassified from Assets Held for Sale Cash reclassified from (to) assets held for sale Represents the portion of cash which has been reclassified from assets held for sale during the reporting period. Net increase (decrease) excluding cash relating to the AutoOne Transaction Represents net increase (decrease) in cash and cash equivalents after providing for adjustment for cash related to discontinued operations. Net (decrease) increase excluding cash relating to the AutoOne Transaction Cash and Cash Equivalents Period Increase Decrease Excluding Discontinued Operations Cash and Cash Equivalents The net realized gains or losses on repurchases of debt and other gains and losses not separately disclosed during the period. Net other realized losses Other Realized Gain (Loss) Net realized loss on settlement of interest rate swap Realized Loss on Settlement of Derivatives, Net Represents the net realized (gain) loss on settlement of interest rate swap. The cash inflow associated with maturities of fixed maturity investments classified as trading. Maturities of fixed maturity investments Proceeds from Maturities of Trading Securities Debt Sales of fixed maturity investments The cash inflow associated with the sale of fixed maturity investments classified as trading. Proceeds from Sale of Trading Securities Debt The cash inflow associated with the sale of equity securities classified as trading. Proceeds from Sale of Trading Common Equity Securities Sales of common equity securities Sales of convertible fixed maturity investments The cash inflow associated with the sale of convertible fixed maturity investments classified as trading. Proceeds from Sale of Trading Securities Convertible Fixed Maturity Investments Purchases of fixed maturity investments Payments to Acquire Trading Securities Debt The cash outflow to acquire fixed maturity investments classified as trading. The cash outflow to acquire common equity securities classified as trading. Payments to Acquire Trading Common Equity Securities Purchases of common equity securities Payments to Acquire Trading Securities Convertible Fixed Maturity Investments Purchases of convertible fixed maturity investments The cash outflow to acquire convertible fixed maturity investments classified as trading. Net change in unsettled investment purchases and sales The net change during the reporting period in accounts receivable on unsettled investment sales and accounts payable on unsettled investment purchases. Increase (Decrease) in Unsettled Accounts Receivable and Accounts Payable Net change in unsettled investment purchases and sales Deferred Policy Acquisition Costs No Longer Deferrable Deferred acquisition costs that are no longer deferrable Represents deferred acquisition costs determined to be no longer deferrable upon prospective adoption of ASU 2010-26. Represents the major number of underwriting units after the aggregation of the reporting segment. Number of Aggregated Major Underwriting Units of Segment Number of aggregated major underwriting units of segment Represents the amount of dividend paid to parent entity. Cash dividends paid to OneBeacon U.S. Enterprises Holdings, Inc. Payments Of Dividend To Parent Interboro LLC [Member] Interboro LLC Represents the details pertaining to Interboro LLC. Reserve for Losses and Loss Adjustment Expenses, Net Net loss and LAE reserves Represents net aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) as of the balance sheet date; grouped amount of all the liabilities associated with the company's insurance policies Schedule of Selected Underwriting Results [Table Text Block] Schedule of net written premiums, earned premiums and ratios for Holdings' insurance operations by segment and in total, as well as for the three major underwriting units within Specialty Insurance Operations Tabular disclosure of selected underwriting results for each reportable segment. Defined Benefit Plan Assumptions Used in Calculating Revaluation Discount Rate Discount rate utilized for revaluation (as a percent) Represents the interest rate used to adjust for the re-evaluated value of money for the plan. Defined Benefit Plan Recognized Net Gain (Loss) Due to Settlements and Remeasurement through Income before Tax Recognized loss due to settlement and re-measurement through pre-tax income Represents the amount of net gain (loss) due to settlements and re-measurements recognized through pre-tax income. Defined Benefit Plan Recognized Net Gain (Loss) Due to Settlements and Remeasurement through other Comprehensive Income Recognized loss due to settlement and re-measurement through other comprehensive income Represents the amount of net gain (loss) due to settlements and re-measurements recognized through other comprehensive income. Transition Services Agreement Initial Term Initial term of Transition Services Agreement Represents the initial term of Transition Services Agreement entered into between OB Holdings and Armour. Discontinued Operation Underwriting Losses due to Adverse Prior Period Loss Reserve Related to Legacy Assumed Reinsurance Treaty Net of Tax After tax underwriting losses Represents the after tax underwriting losses recorded related to adverse prior year loss reserve development related to a legacy assumed reinsurance treaty which were included in discontinued operations. Reserve for Losses and Loss Adjustment Expenses Net Including Portion Attributable to Discontinued Operations Net loss and LAE reserves Represents net aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) including portion attributable to discontinuing operations as of the balance sheet date; grouped amount of all the liabilities associated with the company's insurance policies. Essentia Represents information pertaining to Essentia Insurance Company. Essentia Insurance Company [Member] Subsequent Event [Member] Subsequent Event Identifies event that occurred after the balance sheet date but before financial statements are issued or available to be issued. Premiums Written Net as Percentage of Consolidated Written Premiums Written premiums as a percentage of consolidated written premiums Represents the written premiums as a percentage of consolidated written premiums. One Beacon Insurance Group LLC [Member] OneBeacon Represents the details of information pertaining to OneBeacon Insurance Group LLC. Reinsurance Receivables Incurred but Not Reported Claims in Event of Closure of Transaction Reinsurance recoverable on unpaid losses in the event when runoff transaction closes The portion of reinsurance receivables representing the aggregate amount of estimated amounts, in the event of closure of transaction, due as of the balance sheet date from another insurer; also called reinsurance recoverables or unpaid losses recoverable, that are expected based upon statistical projections of incurred claims and claims settlement expenses which have not yet been reported to the insurer. Schedule of Reinsurance Recoverables Reported as Held for Sale and AM Best Rating [Table Text Block] Schedule of recoverable amounts, the percentage of total reinsurance recoverables reported as held for sale and the reinsurers' A.M. Best Company, Inc. Tabular disclosure of recoverable amounts, the percentage of total reinsurance recoverables reported as held for sale excluding purchase accounting adjustment and the reinsurers' A.M. Best Company, Inc. ("A.M. Best") ratings. Represents the period during which occurrence of resolution is reasonably possible. Reasonably possible period during which resolution could occur (in months) Reasonably Possible Period Resolution Convertible Debt Securities [Member] Convertible fixed maturity investments Asset-backed Securities [Member] Total asset-backed securities Asset-backed securities Assets Held-for-sale, at Carrying Value Assets held for sale CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Cash and Cash Equivalents, at Carrying Value Cash Cash balance at beginning of period Cash balance at end of period Interest paid Interest Paid Ceded Premiums Written Premiums ceded under quota share agreement Net change in ceded reinsurance payable Increase (Decrease) in Reinsurance Payables Net maturities, purchases and sales of short-term investments Payments for (Proceeds from) Short-term Investments Increase (Decrease) in Operating Capital [Abstract] Other operating items: Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Common Class B [Member] Class B common Common Stock, Shares Authorized Common shares, authorized shares (in shares) Common Stock, Shares, Issued Common shares, issued shares (in shares) Common Stock, Shares, Outstanding Common shares, outstanding shares (in shares) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive (loss) income attributable to OB Holdings' common shareholder Condensed Financial Information of Parent Company Only Disclosure [Text Block] SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT Corporate Debt Securities [Member] Corporate Debt Securities Debt securities issued by corporations Asset-backed Securities, Securitized Loans and Receivables [Member] Total other asset-backed securities Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Net unrealized foreign currency translation losses Debt and Capital Lease Obligations Total debt Senior Notes Debt Disclosure [Text Block] Debt Debt Instrument, Face Amount Senior unsecured notes, at face value Debt Instrument, Interest Rate, Effective Percentage Effective yield (as a percent) Debt Instrument, Interest Rate, Stated Percentage Interest rate (as a percent) Debt Instrument, Name [Domain] Debt Instrument, Unamortized Discount Unamortized original issue discount Debt Instrument [Axis] Debt Instrument [Line Items] Debt Schedule of Long-term Debt Instruments [Table] Deferred Income Tax Expense (Benefit) Deferred income tax expense (benefit) Deferred Policy Acquisition Costs Deferred acquisition costs Deferred Policy Acquisition Cost, Amortization Expense Acquisition Amortization Expense Deferred Tax Assets, Net Net deferred tax asset Deferred Tax Assets, Operating Loss Carryforwards, Foreign Net operating loss carryforwards Deferred Tax Assets, Tax Credit Carryforwards, Foreign Foreign tax credit carryforwards Deferred Tax Assets, Tax Credit Carryforwards, Other Net operating loss carryforwards for OBH Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Discontinued Operations Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Loss on sale of discontinued operation, pre-tax Gain (loss) on sale of discontinued operation, pre-tax Pre-tax gain on sale upon closing the transaction Pre-tax loss on sale of discontinued operation Disposal Groups, Including Discontinued Operations, Name [Domain] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Axis] Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Loss from sale of discontinued operations, net of tax Net loss from sale of discontinued operations Gain (loss) on sale of discontinued operation, after tax After tax gain on sale upon closing the transaction Gain (loss) on sale of discontinued operation, after tax Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax Loss from discontinued operations, net of tax Net loss from discontinued operations Gain (loss) on sale of discontinued operation, after tax Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Acquisitions and Dispositions Discontinued Operations Discontinued Operation, Provision for Loss (Gain) on Disposal, Net of Tax Gain (loss) from sale of business, net of tax Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Early Repayment of Senior Debt Repurchases of debt Aggregate purchase price of senior notes purchased and retired Effective Income Tax Rate, Continuing Operations Effective tax rates (as a percent) Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate U.S. statutory rate (as a percent) Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost Expense recognized Equity Securities [Member] Equity Securities Common equity securities Total incurred loss and LAE from continuing Operations Loss and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Loss and LAE Loss and loss adjustment expenses Other Underwriting Expense Other underwriting expenses Other underwriting expenses Extinguishment of Debt, Amount Aggregate principal amount of senior notes purchased and retired Extinguishment of Debt [Axis] Fair Value of Financial Instruments, Policy [Policy Text Block] Fair Value Measurements and Disclosures Foreign Government Debt Securities [Member] Foreign Government Debt Securities Foreign government obligations Net realized and unrealized investment (gains) losses Net realized and unrealized investment (losses) gains Gain (Loss) on Investments Net realized and unrealized investment gains (losses) Gain (Loss) on Investments [Abstract] Gross unrealized gains and losses Net realized gain on Personal Lines Transaction Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal Defined Benefit Plan, Recognized Net (Gain) Loss Due to Settlements Settlement expense Gains (Losses) on Extinguishment of Debt Loss on retirement of debt General and Administrative Expense General and administrative expenses General and administrative expenses CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME Income (Loss) from Continuing Operations, Per Basic Share Net income attributable to OneBeacon's common shareholders per share Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Net loss from discontinued operations, net of tax Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax Pre-tax loss Income Tax Disclosure [Text Block] Income Taxes Income Taxes Paid, Net Net tax payments to state and national governments Total incurred loss and LAE from continuing Operations Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year Loss and LAE incurred relating to Current year losses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years Loss and LAE incurred relating to Prior year losses Reserves for Unpaid Losses and Loss Adjustment Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Interest Expense, Debt Interest expense on debt Interest expense on debt Investment Income, Investment Expense Less investment expenses Investments Total investments Investments [Abstract] Investment securities: Liabilities Total liabilities Liabilities [Abstract] Liabilities Liabilities held for sale Liabilities and Equity Total liabilities, OB Holdings' common shareholder's equity and noncontrolling interests Liabilities of Assets Held-for-sale Liabilities held for sale Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans (as a percent) Long-term Debt Debt Increase (Decrease) in Loss and Loss Adjustment Expense Reserve Net change in loss and LAE reserves Net change in loss and LAE reserves Common shares and paid-in surplus (par value $001; authorized, 200,000,000 shares; issued and outstanding, [95,068,457] and 94,416,477 shares) Stockholders' Equity Attributable to Noncontrolling Interest Total noncontrolling interests Total noncontrolling interests Noncontrolling Interest, Ownership Percentage by Parent Ownership Interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Distributions Contributions Noncontrolling Interest, Increase from Equity Issuance or Sale of Parent Equity Interest Collateralized Mortgage Backed Securities [Member] Total mortgage-backed securities Net cash provided from operations - continuing operations Net Cash Provided by (Used in) Operating Activities, Continuing Operations Net cash used for financing activities Net Cash Provided by (Used in) Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Cash flows from financing activities: Net cash provided from investing activities Net Cash Provided by (Used in) Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Cash flows from investing activities: Net cash used for operations Net Cash Provided by (Used in) Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Cash flows from operations: Settlement of interest rate swap Payments for (Proceeds from) Hedge, Financing Activities Net (loss) income attributable to OB Holdings' common shareholder Net Income (Loss) Available to Common Stockholders, Basic Net income attributable to OneBeacon's common shareholders Cash and Cash Equivalents, Period Increase (Decrease)