Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Summary of performance share activity 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 shares outstanding and accrued expense for performance shares awarded under the OB Incentive Plan Schedule of Performance Shares Outstanding and Accrued Expense by Performance Cycle under Incentive 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 restricted shares activity Schedule of Nonvested Restricted Share Activity [Table Text Block] Tabular disclosure of the changes in nonvested restricted shares and the unamortized grant date fair value. Stockholders' Equity Note [Abstract] Class of Treasury Stock [Table] Class of Treasury Stock [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Total OneBeacon common shareholders' equity Parent [Member] Accum. other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss) [Member] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Class A common Common Class A [Member] Common Stock Common Stock [Member] Equity, Class of Treasury Stock [Line Items] Equity, Class of Treasury Stock [Line Items] Stock Repurchase Program, Authorized Amount Stock Repurchase Program, Authorized Amount Repurchase and retirement of common shares Stock Repurchased and Retired During Period, Value Dividends Dividends, Common Stock, Cash Dividends, per share, cash paid Common Stock, Dividends, Per Share, Cash Paid Pre-tax changes to accumulated other comprehensive income Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Insurance Loss Reserves [Abstract] Unpaid Loss and Loss Adjustment Expense (LAE) Reserves Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Discontinued Operations and Disposal Groups [Abstract] Schedule of discontinued operations balance sheet statement of operations and other disclosures Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet and Additional Disclosures [Table Text Block] Reinsurers Ceded Credit Risk [Table Text Block] Segment Reporting [Abstract] Financial information for OneBeacon's segments Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of selected balance sheet of the segments Schedule of Selected Balance Sheet Data by Segment [Table Text Block] Tabular disclosure of selected balance sheet data for each reportable segment. Schedule of net written premiums, earned premiums and ratios for OneBeacon's insurance operations by segment and in total, as well as for the three major underwriting units within Specialty Insurance Operations Schedule of selected underwriting results [Table Text Block] Tabular disclosure of selected underwriting results for each reportable segment. Statement of Financial Position [Abstract] Common shares, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common shares, authorized shares (in shares) Common Stock, Shares Authorized Common shares, issued shares (in shares) Common Stock, Shares, Issued Common shares, outstanding shares (in shares) Common Stock, Shares, Outstanding Discontinued Operations Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Loss and Loss Adjustment Expense Reserves [Table] Loss and Loss Adjustment Expense Reserves [Table] Schedule of loss and loss adjustment expense ("LAE") reserve activities. Disposal Group Name [Axis] Disposal Group Name [Axis] Disposal Groups, Including Discontinued Operations, Name [Domain] Disposal Groups, Including Discontinued Operations, Name [Domain] Runoff Runoff [Member] Represents Runoff which provides products and services to automobile assigned risk markets. Loss and Loss Adjustment Expense Loss and Loss Adjustment Expense [Line Items] Loss and Loss Adjustment Expense Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Gross beginning balance Liability for Claims and Claims Adjustment Expense Less beginning reinsurance recoverable on unpaid losses Reinsurance Recoverables, Incurred but Not Reported Claims Loss and LAE incurred relating to Current year losses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Current Year Loss and LAE incurred relating to prior year losses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years Total incurred loss and LAE from continuing operations Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Loss and LAE paid relating to Current year losses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Loss and LAE paid relating to Prior year losses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total loss and LAE payments from continuing operations Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Net ending loss and LAE reserves Liability for Unpaid Claims and Claims Adjustment Expense, Net Plus ending reinsurance recoverable on unpaid losses Gross ending balance Debt Disclosure [Abstract] Schedule of debt outstanding Schedule of Long-term Debt Instruments [Table Text Block] Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Common shares and paid-in surplus Common Stock Including Additional Paid in Capital [Member] Retained earnings Retained Earnings [Member] Noncontrolling interests, after tax Noncontrolling Interest [Member] Statement Statement [Line Items] Increase (Decrease) in Stockholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Balances Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net change in benefit plan assets and obligations, net of tax Total comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Amortization of restricted share awards Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Issuance of common shares Stock Issued During Period, Value, New Issues Repurchase and retirement of common shares Dividends, Common Stock, Cash Balances Schedule of Investments [Abstract] Schedule of Trading Securities and Other Trading Assets [Table] Trading Securities [Table] Disposal Group Name [Axis] Statement, Operating Activities Segment [Axis] Operating Activities [Axis] Segment, Operating Activities [Domain] Operating Activities [Domain] Segment, Discontinued Operations Discontinued Operations [Member] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Trading Securities and Assets [Domain] Major Types of Debt and Equity Securities [Domain] Common equity securities Convertible fixed maturity investments Convertible Debt Securities [Member] Other investments Other Long-term Investments [Member] Total common equity securities, convertible fixed maturity investments and other investments Common Equity, Convertible Fixed, and Other Investments [Member] Common Equity, Convertible Fixed, and Other Investments [Member] Schedule of Trading Securities and Other Trading Assets [Line Items] Schedule of Trading Securities and Other Trading Assets [Line Items] Unrestricted Collateral From Customers Unrestricted Collateral From Customers Unrestricted Collateral From Customers Cost or amortized cost Trading Securities, Cost Trading Securities Gross Unrealized Gain Trading Securities Gross Unrealized Gain The gross amount of unrealized gains for trading securities as of the balance sheet date. Trading Securities Gross Unrealized Loss Trading Securities Gross Unrealized Loss The gross amount of unrealized losses for trading securities as of the balance sheet date. Net unrealized foreign currency translation 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. Carrying value Trading Securities, Debt Total investment securities Investments Income Statement [Abstract] Statement, Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Revenues Revenues [Abstract] Earned premiums Premiums Earned, Net Net investment income Net Investment Income Net realized and change in unrealized investment gains Gain (Loss) on Investments Net other revenues Other Nonoperating Income (Expense) Total revenues Revenues Expenses Benefits, Losses and Expenses [Abstract] Loss and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net 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. Other underwriting expenses Other Underwriting Expense General and administrative expenses General and Administrative Expense Interest expense Interest Expense, Debt Total expenses Benefits, Losses and Expenses Pre-tax income (loss) from continuing operations Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax (expense) benefit Income Tax Expense (Benefit) Net income (loss) from continuing operations Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest Loss from discontinued operations, net of tax Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax Net income (loss), including noncontrolling interests Less: Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Net income (loss) attributable to OneBeacon's common shareholders Net Income (Loss) Available to Common Stockholders, Basic Other comprehensive income, net of tax Comprehensive income attributable to OneBeacon's common shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Earnings (loss) per share attributable to OneBeacon's common shareholders—basic and diluted Earnings Per Share [Abstract] Net income from continuing operations, per share Income (Loss) from Continuing Operations, Per Basic and Diluted Share Loss from discontinued operations, net of tax, per share Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic and Diluted Share Net income (loss) attributable to OneBeacon's common shareholders per share Earnings Per Share, Basic and Diluted Dividends declared and paid per OneBeacon's common share Business Combinations [Abstract] Acquisitions and Dispositions Mergers, Acquisitions and Dispositions Disclosures [Text Block] Fair Value Inputs Assets Quantitative Information [Table] Fair Value Inputs, Assets, Quantitative Information [Table] Credit Rating, Standard & Poor's [Axis] Credit Rating, Standard & Poor's [Axis] External Credit Rating, Standard & Poor's [Domain] External Credit Rating, Standard & Poor's [Domain] Standard & Poor's, AA+ Rating [Member] Standard & Poor's, AA+ Rating [Member] Standard Poors N R Rating [Member] Standard Poors N R Rating [Member] NR credit rating as defined by the external credit rating agency, Standard & Poor's. Standard & Poor's, AAA Rating [Member] Standard & Poor's, AAA Rating [Member] Fair Value by Asset Class [Axis] Asset Class [Axis] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class [Domain] Asset Class [Domain] Commercial Commercial Mortgage Backed Securities [Member] Non-agency RMBS Residential Mortgage Backed Securities [Member] Total Non-agency Mortgage-backed Securities, Issued by Private Enterprises [Member] Mortgage-backed and asset-backed securities Asset-backed Securities [Member] Preferred stocks Preferred Stock [Member] Valuation Technique [Axis] Valuation Technique [Axis] Valuation technique [Domain] Valuation Technique [Domain] Discounted cash flow Discounted Cash Flow Valuation Technique [Member] Represents the discounted cash flow technique used to measure fair value. Broker Pricing [Member] Broker Pricing [Member] Valuation for level 3 investments based on broker pricing. Significant unobservable inputs Fair Value Inputs, Assets, Quantitative Information [Line Items] Assets, Fair Value Disclosure Assets, Fair Value Disclosure Discount Rate Fair Value Inputs, Discount Rate Default rate Fair Value Inputs, Loss Severity Number of Securities in each Asset type Number of Securities in each Asset type Number of Securities in each Asset Type Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table] Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Earnings (loss) attributable to OneBeacon's common shareholders—basic and diluted (in millions): Earnings Per Share, Basic and Diluted [Abstract] Net income (loss) from continuing operations attributable to OneBeacon's common shareholders Income (Loss) from Continuing Operations Attributable to Parent Allocation of loss (income) for participating unvested restricted common shares Income (Loss) Allocated to Participating Unvested Restricted Common Shares Represents the portion of income (loss) allocated to participating unvested restricted common shares. Dividends paid on participating restricted common shares Dividends, Share-based Compensation Total allocation to restricted common shares Aggregate Allocation to Restricted Shares Including Dividend Represents the total amount allocated to restricted shares including dividend on restricted stock. Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts 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. Undistributed Earnings, Basic [Abstract] Undistributed Earnings, Basic [Abstract] Net income (loss) from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts Dividends paid, 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. Total undistributed (overdistributed) net earnings, net of restricted common share amounts Undistributed Earnings, Basic Weighted Average Number of Shares Outstanding, Basic [Abstract] Total weighted average common shares outstanding Weighted Average Number of Shares Issued, Basic Weighted average unvested restricted common shares Weighted Average Number of Shares, Restricted Stock Basic and diluted earnings (loss) per share denominator Weighted Average Number of Shares Outstanding, Basic Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract] Net income from continuing operations attributable to OneBeacon's common shareholders Income (Loss) from Continuing Operations, Per Basic Share Dividends declared and paid Undistributed (overdistributed) earnings Earnings Per Share, Basic, Undistributed Organization, Consolidation and Presentation of Financial Statements [Abstract] Consolidating Balance Sheet Condensed Balance Sheet [Table Text Block] Consolidating Statement of Operations and Comprehensive (Loss) Income Condensed Income Statement [Table Text Block] Consolidating Statement of Cash Flows Condensed Cash Flow Statement [Table Text Block] Compensation and Retirement Disclosure [Abstract] Retirement Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Reinsurance Disclosures [Abstract] Reinsurance Retention Policy [Table] Reinsurance Retention Policy [Table] Standard & Poor's, AA Rating [Member] Standard & Poor's, AA Rating [Member] Standard & Poor's, A Rating [Member] Standard & Poor's, A Rating [Member] Standard & Poor's, BBB+ Rating [Member] Standard & Poor's, BBB+ Rating [Member] Insurance Product Line [Axis] Insurance Product Line [Axis] Insurance Product Line [Domain] Insurance Product Line [Domain] Reinsurance Type and Tier Identifier [Axis] Reinsurance Type and Tier Identifier [Axis] Reinsurance Type and Tier Identifier [Axis] Reinsurance Type and Tier Identifier [Domain] Reinsurance Type and Tier Identifier [Domain] [Domain] for Reinsurance Type and Tier Identifier [Axis] Layer Three [Member] Layer Three [Member] Layer Three [Member] Reinsurance Policy, Type [Axis] Reinsurance Policy, Type [Axis] Reinsurance Policy [Domain] Reinsurance Policy [Domain] Property Catastrophe Reinsurance [Member] Property Catastrophe Reinsurance [Member] Property Catastrophe Reinsurance [Member] Property Per Risk Reinsurance Program [Member] Property Per Risk Reinsurance Program [Member] Property Per Risk Reinsurance Program [Member] MPCI Portfolio [Member] MPCI Portfolio [Member] MPCI portfolio Crop-Hail Portfolio [Member] Crop-Hail Portfolio [Member] Crop-hail portfolio Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum Maximum [Member] Reinsurance Retention Policy [Line Items] Reinsurance Retention Policy [Line Items] Reinsurance Retention Policy, Amount Retained Reinsurance Retention Policy, Amount Retained Reinsurance Retention Policy, Excess Retention, Amount Reinsured Reinsurance Retention Policy, Excess Retention, Amount Reinsured Coverage in Excess of Retained Amount Coverage in Excess of Retained Amount The percent of losses reinsured in excess of the retained amount. Retained Losses Retained Losses Percent of losses retained by entity in connection with reinsurance treaties. Reinsurance recoverable on paid losses Reinsurance Receivables Reinsurance recoverable on unpaid losses Reinsurance recoverables Reinsurance Recoverables Percentage of total paid and unpaid reinsurance recoverables Reinsurance Recoverables, Percentage of Total Number of ratings in A.M. Best ratings AM Best Rating System Number of Ratings The number of ratings in the A.M. Best rating system. Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return on Plan Assets Amortization of unrecognized loss Defined Benefit Plan, Amortization of Gains (Losses) Net periodic pension income before special termination benefits 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. Special termination benefits expense(1) Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements Total net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost Contributions to non-qualified plan in 2014 Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year Pension contributions Pension Contributions Schedule of Condensed Financial Statements [Table] Condensed Financial Statements [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] The Company (guarantor) Parent Company [Member] Non-guarantor subsidiaries Non-Guarantor Subsidiaries [Member] OBH (issuer) Subsidiary Issuer [Member] Consolidating adjustments Consolidation, Eliminations [Member] Condensed Financial Statements, Captions [Line Items] Condensed Financial Statements, Captions [Line Items] Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Net income (loss) including noncontrolling interests Charges (credits) to reconcile net income to cash flows (used for) provided from operations: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Undistributed earnings from subsidiaries Income (Loss) from Equity Method Investments Net loss from discontinued operations Net loss from sale of discontinued operations Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax Net realized and change in unrealized investment gains Net realized gain on sale of business Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal Deferred income tax expense Deferred Income Tax Expense (Benefit) Dividends received from subsidiaries Cash Dividends Paid to Parent Company by Consolidated Subsidiaries Other operating items: Net Cash Provided by (Used in) Operating Activities [Abstract] Net change in loss and LAE reserves Increase (Decrease) in Loss and Loss Adjustment Expense Reserve Net change in unearned premiums Increase (Decrease) in Unearned Premiums Net change in premiums receivable Increase (Decrease) in Premiums Receivable Net change in reinsurance recoverable on paid and unpaid losses Increase (Decrease) in Reinsurance Recoverable Net change in other assets and liabilities Increase (Decrease) in Other Operating Assets and Liabilities, Net Net cash provided from operations—continuing operations Net Cash Provided by (Used in) Operating Activities, Continuing Operations Net cash used for operations—discontinued operations Cash Provided by (Used in) Operating Activities, Discontinued Operations Net cash provided from (used for) operations Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Net maturities, purchases and sales of short-term investments Payments for (Proceeds from) Short-term Investments Maturities of fixed maturity investments Proceeds from Maturities of Trading Securities Debt The cash inflow associated with maturities of fixed maturity investments classified as trading. Sales of fixed maturity investments Proceeds from Sale of Trading Securities Debt The cash inflow associated with the sale of fixed maturity investments classified as trading. Sales of common equity securities Proceeds from Sale of Trading Common Equity Securities The cash inflow associated with the sale of equity securities classified as trading. Sales of convertible fixed maturity investments Proceeds from Sale of Trading Securities Convertible Fixed Maturity Investments The cash inflow associated with the sale of convertible fixed maturity investments classified as trading. Distributions and redemptions of other investments Proceeds from Sale and Maturity of Other Investments Purchases of fixed maturity investments Payments to Acquire Trading Securities Debt The cash outflow to acquire fixed maturity investments classified as trading. Purchases of common equity securities Payments to Acquire Trading Common Equity Securities The cash outflow to acquire common equity securities classified as trading. Purchases of convertible fixed maturity investments Payments to Acquire Trading Securities Convertible Fixed Maturity Investments The cash outflow to acquire convertible fixed maturity investments classified as trading. Contributions for other investments Payments to Acquire Other Investments Proceeds from sale of business Proceeds from Divestiture of Businesses Net change in unsettled investment purchases and sales Increase (Decrease) in Unsettled Accounts Receivable and Accounts Payable The net change during the reporting period in accounts receivable on unsettled investment sales and accounts payable on unsettled investment purchases. Net acquisitions of property and equipment Payments to Acquire Property, Plant, and Equipment Return of capital to OneBeacon U.S. Enterprises Holdings, Inc. Cash Paid to Parent Company by Consolidated Subsidiaries, Return of Capital Cash Paid to Parent Company by Consolidated Subsidiaries, Return of Capital Net cash provided from investing activities—continuing operations Net Cash Provided by (Used in) Investing Activities, Continuing Operations Net cash provided from investing activities—discontinued operations Cash Provided by (Used in) Investing Activities, Discontinued Operations Net cash provided from investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Cash dividends paid to common shareholders Payments of Ordinary Dividends, Common Stock Cash dividends paid to parent Cash Dividends Paid to Parent Company Return of capital to OneBeacon U.S. Enterprises Holdings, Inc. Payments for Repurchase of Common Stock Proceeds on sale of OB Holdings Lux Proceeds from Sale of Subsidiary The cash inflow associated with the amount received from the sale of a subsidiary during the period. Net cash used for financing activities—continuing operations Net Cash Provided by (Used in) Financing Activities, Continuing Operations Net cash used for financing activities—discontinued operations Cash Provided by (Used in) Financing Activities, Discontinued Operations Net cash used for financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash during period Cash and Cash Equivalents, Period Increase (Decrease) Cash balance at beginning of period Cash and Cash Equivalents, at Carrying Value Cash balance at end of period Proceeds from Contributed Capital Proceeds from Contributed Capital Issuance of debt, net of debt issuance costs Proceeds from Issuance of Debt Repayments of Long-term Capital Lease Obligations Repayments of Long-term Capital Lease Obligations U.S. Government and agency obligations US Treasury and Government [Member] Debt securities issued by corporations Corporate Debt Securities [Member] Municipal obligations Municipal Bonds [Member] Foreign government obligations Foreign Government Debt Securities [Member] Total fixed maturity investments Fixed Maturities [Member] Trading Securities Gross Unrealized Loss Net unrealized foreign currency translation gains (losses) Document and Entity Information Document and Entity Information Class B common Common Class B [Member] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date Amendment Flag Amendment Flag Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Investment Securities Investment Holdings [Text Block] Employee Share-Based Incentive Compensation Plans Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 3 Fair Value, Inputs, Level 3 [Member] Fixed maturity investments Short-term investments Short-term Investments [Member] Common equity securities Equity Securities [Member] Unrealized gains (losses) on unsold securities Trading Securities, Change in Unrealized Holding Gain (Loss) Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring basis Fair Value, Measurements, Recurring [Member] Hedge funds Hedge Funds [Member] Private equity funds Private Equity Funds [Member] Community Reinvestment Vehicle [Member] Community Reinvestment Vehicle [Member] Community Reinvestment Vehicle [Member] Trust Preferred Securities Subject to Mandatory Redemption [Member] Trust Certificates Issued Upon Dissolution of a Private Equity Fund [Member] Trust Certificates Issued Upon Dissolution of a Private Equity Fund [Member] Single Fund Single Fund [Member] Single Fund [Member] Tax Development Fund [Member] Tax Development Fund [Member] Tax Development Fund [Member] U.S. Government and agency obligations Municipal obligations Foreign government obligations 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. Preferred stocks Investment Sector Detail [Axis] Investment Sector [Axis] Investment Sector [Domain] Investment Sector [Domain] Consumer Consumer [Member] Represents the consumer sector. Industrial Industrial [Member] Represents the industrial sector. Financial Financials [Member] Represents the financial sector. Communications Communications [Member] Represents the communications sector. Energy Energy [Member] Represents the energy sector. Basic materials Basic Materials [Member] Represents the basic materials sector Utilities Utilities [Member] Represents the utilities sector. Technology Technology [Member] Represents the technology sector. Other Other Sector [Member] Represents all other sectors that are not otherwise covered by the taxonomy. Fair Value Estimate of Fair Value Measurement [Member] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers Change in Deferred Gains (Losses) Change in Deferred Gains (Losses) Changes in deferred gains and losses on sales of investments between entities under common control. Debt securities issued by corporations Investments, Fair Value Disclosure Percentage of investments based on observable inputs Investments, Percent of Total Investments, Percent of Total Percentage of Fair Value Investments Using Observable Inputs 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. Other investments Other Investments Number of Investments Number of Investments Number of Investments Securities reclassified to assets held for sale Fixed Maturities InvestmentsTrading Securities Debt Reclassified to Assets Held-for-sale Fixed Maturities InvestmentsTrading Securities Debt Reclassified to Assets Held-for-sale Percentage of private preferred stock Private Preferred Stock Percentage The percentage of the preferred stock portfolio represented by one private preferred stock. Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] OBH Senior Notes OBH Senior Notes [Member] OBH Senior Notes [Member] 2012 OBH Senior Notes 2012 OBH Senior Notes [Member] 2012 OBH Senior Notes [Member] OBH (issuer) Onebeacon U S Holdings Inc [Member] Represents OneBeacon U.S. Holdings, Inc. Debt Debt Instrument [Line Items] Senior unsecured notes, at face value Debt Instrument, Face Amount Unamortized original issue discount Debt Instrument, Unamortized Discount Senior unsecured notes, carrying value Debt and Capital Lease Obligations Percentage of par value at which debt was issued Debt Instrument Issue Price As Percentage Of Par Value Represents the price of debt issued as a percentage of its par value. Interest rate (as a percent) Debt Instrument, Interest Rate, Stated Percentage Debt issue costs Deferred Finance Costs, Gross Underwriting discount 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. Effective yield (as a percent) Debt Instrument, Interest Rate, Effective Percentage Debt Long-term Debt Commitments and Contingencies Disclosure [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Litigation Status [Axis] Litigation Status [Axis] Litigation Status [Domain] Litigation Status [Domain] Pending Litigation Pending Litigation [Member] Litigation Case [Axis] Litigation Case [Axis] Litigation Case Type [Domain] Litigation Case [Domain] Tribune Company Litigation Tribune Company Litigation [Member] Tribune Company Litigation [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Proceeds common stock, litigation settlement Proceeds Common Stock, Litigation Settlement Proceeds Common Stock, Litigation Settlement Consolidating Financial Information Condensed Financial Information of Parent Company Only Disclosure [Text Block] Gross investment income Interest and Dividend Income, Operating Less investment expenses Investment Income, Investment Expense Net investment income Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Table] Class of Financing Receivable [Axis] Class of Financing Receivable [Axis] Class of Financing Receivable [Domain] Class of Financing Receivable [Domain] Surplus Notes For Sale Of Discontinued Operations [Member] Surplus Notes For Sale Of Discontinued Operations [Member] Surplus Notes For Sale Of Discontinued Operations [Member] Economic Entity [Axis] Economic Entity [Axis] Economic entities which constitute neither defined legal entities nor reportable segments of the reporting entity. Economic Entity [Domain] Economic Entity [Domain] A listing of economic entities which are not legal entities or defined reportable segments of the entity. General Reinsurance Corporation Cover General Reinsurance Corporation Cover [Member] Represents the adverse loss reserve development cover from General Reinsurance Corporation. Ceded Credit Risk, Reinsurer Ceded Credit Risk, Reinsurer [Axis] Ceded Credit Risk, Reinsurer [Domain] Ceded Credit Risk, Reinsurer [Domain] National Indemnity Company (NICO) and General Reinsurance Corporation National Indemnity Company and General Reinsurance Corporation [Member] Represents National Indemnity Company and General Reinsurance Corporation, which are third party reinsurers of the entity. Hanover Insurance Company Hanover Insurance Company [Member] Represents Hanover Insurance Company, which is a third party reinsurer of the entity. Tokio Marine and Nichido Fire Tokio Marine and Nichido Fire [Member] Represents Tokio Marine and Nichido Fire, which are third party reinsurers of the entity. Munich Reinsurance America Munich Reinsurance America [Member] Represents Munich Reinsurance America, which is a third party reinsurer of the entity. Tower Insurance Company Everest Reinsurance [Member] Everest Reinsurance [Member] Reinsurance Contract by Nature [Axis] Reinsurance Contract by Nature [Axis] Information by name that identifies a reinsurance contract by type. Reinsurance Contract Name [Domain] Reinsurance Contract Name [Domain] The name that identifies a reinsurance contract by type. GRC Cover GRC Cover [Member] GRC Cover [Member] NICO Cover 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. Dispositions Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Change In Surplus Notes Issued As Seller Financing In Sale Of Discontinued Operations Change In Surplus Notes Issued As Seller Financing In Sale Of Discontinued Operations Change In Surplus Notes Issued As Seller Financing In Sale Of Discontinued Operations Surplus Notes Issued As Seller Financing In Sale Of Discontinued Operations Surplus Notes Issued As Seller Financing In Sale Of Discontinued Operations Surplus notes issued as seller financing for sale of discontinued operations Income (loss) from discontinued operations, net of tax Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Quota Share Reinsurance Agreement Percentage Quota Share Reinsurance Agreement Percentage Quota Share Reinsurance Agreement Percentage Reinsurance Recoverables, Gross Reinsurance Recoverables, Gross Unpaid loss and loss adjustment expense reserves Pre-tax loss Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax Income (loss) from discontinued operations, net of tax Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Other revenue Other Income Liability for Unpaid Claims and Claims Adjustment Expense, Net Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Discontinued Operations Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Discontinued Operations Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Discontinued Operations Loss from sale of discontinued operations, net of tax Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense Amount, net of tax Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense Amount, net of tax Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense Amount, net of tax Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Adverse loss reserve development on losses occurring in years 2000 and prior 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. Loss reserves ceded at acquisition Loss Reserves Ceded at Acquisition Represents the amount of reserves ceded as of the date of the OneBeacon acquisition. LAE claims, net Liability for Asbestos and Environmental Claims, Net Reinsurance recoverables LAE claims, estimated future losses Liabilities for Asbestos and Environmental Claims, Estimated Future Losses Liabilities for Asbestos and Environmental Claims, Estimated Future Losses Estimated incurred losses ceded Estimated Incurred Losses Ceded Represents the estimated amount of loss ceded as of the balance sheet date. Reinsurance recoverables Reinsurance Recoverables, Guaranteed Reinsurance Recoverables, Guaranteed Earnings per Share Earnings Per Share [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Specialty Products Specialty Products [Member] Major underwriting unit comprised of OBPI, PIM, OBSP and OBES. Specialty Industries Specialty Industries [Member] Major underwriting unit comprised of IMU, OBTI, OBA, OBGR and OBEG. Investing, Financing and Corporate Investing Financing and Corporate Operations [Member] Includes information about OneBeacon segment Investing, Financing and Corporate Operations. Segment Information Segment Reporting Information [Line Items] Number of Major Underwriting Units Number of Major Underwriting Units Number of Major Underwriting Units Net written premiums Premiums Written, Net Loss and loss adjustment expense Policy acquisition expenses Other underwriting expenses Total underwriting income (loss) Underwriting Income (Loss) General and administrative expenses Interest expense Segment Information, Additional Disclosure Segment Reporting Information, Additional Information [Abstract] Reinsurance recoverables Deferred acquisition costs Deferred Policy Acquisition Costs Unearned premiums Unearned Premiums Loss and LAE Ratio Loss and LAE Ratio Loss and LAE Ratio Expense Ratio Expense Ratio Expense Ratio Combined Ratio Combined Ratio Number of underwriting segments Number of Operating Segments Common Shareholders' Equity Stockholders' Equity Note Disclosure [Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Long Term Incentive Plan Long Term Incentive Plan [Member] Long Term Incentive Plan Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Performance shares Performance Shares [Member] Share Based Compensation Arrangement by Performance Shares Award Performance Cycle [Axis] 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. Share Based Compensation Arrangement by Performance Shares Award Performance Cycle [Domain] Share Based Compensation Arrangement by Performance Shares Award Performance Cycle [Domain] This element represents minimum performance period for right to receive performance share from the entity. Performance cycle 2011 - 2013 Range of Performance Cycle from 2011 To 2013 [Member] This element represents period of performance cycle from 2011 to 2013. Performance cycle 2012 - 2014 Range of Performance Cycle from 2012 To 2014 [Member] This element represents period of performance cycle from 2012 to 2014. Performance cycle 2013 - 2015 Range of Performance Cycle from 2013 To 2015 [Member] Range of Performance Cycle from 2013 To 2015 [Member] Performance cycle 2014 - 2016 Range of Performance Cycle from 2014 To 2016 [Member] Range of Performance Cycle from 2014 To 2016 [Member] Performance cycle 2012 - 2016 Performance Cycle 2012 - 2016 [Member] Range of performance cycle 2012 - 2016. Performance Cycle 2010 To 2012 Range of Performance Cycle from 2010 To 2012 [Member] This element represents period of performance cycle from 2010 to 2012. Employee Share-Based Incentive Compensation Plans Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Activity in plan Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Payments and deferrals (in shares) Share Based Compensation Arrangement by 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. New awards (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Forfeitures and net change in assumed forfeitures (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period End of period (in shares) Expense amortized Accrued Expense Roll Forward [Abstract] Accrued expense at beginning of period Incentive Compensation Payable Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. Payments and deferrals 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. Forfeitures and net change in assumed forfeitures 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. Expense recognized Allocated Share-based Compensation Expense Accrued expense at end of period Additional disclosures Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract] Growth target percentage Share-based Compensation Arrangement by Share-based Payment Award, Payout for Minimum Growth Target Percentage Share-based Compensation Arrangement by Share-based Payment Award, Payout for Minimum Growth Target Percentage Target OB Performance Shares outstanding 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. Assumed forfeitures (in shares) 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 Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Assumed Forfeitures Payable Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Assumed Forfeitures Payable Additional compensation cost that would be recognized if all outstanding performance shares vested 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. AA A BBB Standard & Poor's, BBB Rating [Member] BB Standard & Poor's, BB Rating [Member] Other Other Standard Poors Rating [Member] Remainder of ratings not separately disclosed. Discontinued Operations Loss attributable to OneBeacon's common shareholders—basic and diluted (in millions): Net loss from discontinued operations attributable to OneBeacon's common shareholders Allocation of loss for participating unvested restricted common shares Loss per share denominator—basic and diluted (in millions): Total weighted average common shares outstanding Basic and diluted earnings (loss) per share denominator Loss per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars): Earnings Per Share, Basic [Abstract] Loss from discontinued operations attributable to OneBeacon's common shareholders per share Accounting Policies [Abstract] Recently Adopted Changes in Accounting Principles New Accounting Pronouncements, Policy [Policy Text Block] Reinsurance Reinsurance [Text Block] Investments, Debt and Equity Securities [Abstract] Residential, Prime, Financing Receivable [Member] Residential, Prime, Financing Receivable [Member] Residential, Subprime, Financing Receivable [Member] Residential, Subprime, Financing Receivable [Member] Security Issuance Year [Axis] Security Issuance Year [Axis] Represents the year of issuance for securities held by the entity. Security Issuance Year [Domain] Security Issuance Year [Domain] Represents the respective year in which the security was issued. Securities Issued in 2004 Securities Issued in 2004 [Member] Securities Issued in 2004 [Member] Securities Issued in 2005 Securities Issued in 2005 [Member] Securities Issued in 2005 [Member] Securities Issued in 2006 Securities Issued in 2006 [Member] Securities Issued in 2006 [Member] Securities issued in 2007 Securities Issued in 2007 [Member] Represents securities issued in 2007. Securities Issued in 2008 Securities Issued in 2008 [Member] Securities Issued in 2008 [Member] Securities Issued in 2009 [Member] Securities Issued in 2009 [Member] Securities issued in 2009 Securities issued in 2010 Securities Issued in 2010 [Member] Represents securities issued in 2010. Securities issued in 2011 Securities Issued in 2011 [Member] Represents securities issued in 2011. Securities issued in 2012 Securities Issued in 2012 [Member] Represents securities issued in 2012. Securities Issued in 2013 Securities Issued in 2013 [Member] Securities Issued in 2013 [Member] Securities Issued in 2014 [Member] Securities Issued in 2014 [Member] Represents securities issued in 2014. Investment by Tranche Levels [Axis] Investment by Tranche Levels [Axis] Information by tranche level of non-agency RMBS securities and non-agency CMBS securities. Investment Tranche Levels [Domain] Investment Tranche Levels [Domain] Tranche level indicators related to investments. Super Senior Super Senior [Member] This element represent the tranche level of Super Senior, which were rated AAA and were senior to other AAA bonds Senior Senior [Member] This element represent the tranche level of Senior, which were rated AAA and were senior to non-AAA bonds. Subordinate Subordinate [Member] This element represent the tranche level of Subordinate, which were not rated AAA and were junior to AAA bonds. Commercial Mortgage Backed Securities Floating Rate Commercial Mortgage Backed Securities Floating Rate [Member] Securities collateralized by commercial real estate mortgage loans which have a floating interest rate. Commercial Mortgage Backed Securities Fixed Rate Commercial Mortgage Backed Securities Fixed Rate [Member] Securities collateralized by commercial real estate mortgage loans which have a fixed interest rate. Total Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Table] Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Table] White Mountains Insurance Group Ltd White Mountains Insurance Group Ltd [Member] Represents White Mountains Insurance Group Ltd. Basis of presentation Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items] Participation percentage in quota share reinsurance agreements Participation Percentage in Quota Share Reinsurance Agreements Represents the participation percentage in quota share reinsurance agreements. Ownership interest (as a percent) Noncontrolling Interest, Ownership Percentage by Parent Unamortized discount Net investment income, pre-tax Investment Income [Table Text Block] Net realized and unrealized investment gains (losses), pre-tax Realized Gain (Loss) on Investments [Table Text Block] Net unrealized investment gains (losses) Unrealized Gain (Loss) on Investments [Table Text Block] Gross unrealized investment gains or losses Trading Securities, Gross Unrealized Gain (Loss) [Table Text Block] The net amount of gross unrealized gains (losses) for securities, at a point in time, which are categorized as trading securities. Investment holdings, fixed maturity investments 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, common equity securities, convertible fixed maturity investments and other investments Trading Securities [Table Text Block] Fair value measurements by level, investment securities Fair Value, Assets Measured on Recurring Basis [Table Text Block] Debt securities, credit ratings Investment Fixed Maturity Securities Credit Quality Indicators [Table Text Block] Tabular disclosure of fixed maturity securities by credit quality indicator. Fair Value Assets Measured on Recurring Basis Measurement Input Reconciliation 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. Fair value, significant unobservable inputs Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Net unrealized gains or losses for Level 3 investments 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. Mortgage-backed and asset-backed securities Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS 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. Non-agency RMBS, collateral quality and tranche levels 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 CMBS, type of interest rate 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. Other investments 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. Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds 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 private equity funds subject to lock-up periods Private Equity Lock up Period [Table Text Block] Tabular disclosure of expected lock up period of private equity investments. Legal Contingencies Commitments and Contingencies Disclosure [Text Block] Schedule of components of net periodic benefit cost (income) Schedule of Net Benefit Costs [Table Text Block] Level 2 Investment securities Other Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Amortization/accretion Accretion (Amortization) of Discounts and Premiums, Investments Total net realized and unrealized gains (losses) 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. Purchases Payments to Acquire Investments Sales Proceeds from Sale and Maturity of Trading Securities Held-for-investment Transfers in 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 out 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. Fair Value Assets Measured on Recurring Basis Transfers out Recategorized Fair Value Assets Measured on Recurring Basis Transfers out Recategorized Fair Value Assets Measured on Recurring Basis Transfers out Recategorized Debt Debt Disclosure [Text Block] Segment Information Segment Reporting Disclosure [Text Block] Income Tax Disclosure [Abstract] Income Tax Contingency [Table] Income Tax Contingency [Table] Tax Assessment Year [Axis] Tax Assessment Year [Axis] Tax Assessment Year [Axis] Tax Assessment Year [Domain] Tax Assessment Year [Domain] [Domain] for Tax Assessment Year [Axis] Tax Year 2005 to 2006 [Member] Tax Year 2005 to 2006 [Member] Tax Year 2005 to 2006 [Member] Tax Assessment Year 2007 to 2009 Tax Assessment Year 2007 to 2009 [Member] Tax Assessment Year 2007 to 2009 [Member] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Foreign Tax Authority Foreign Tax Authority [Member] Internal Revenue Service (IRS) Internal Revenue Service (IRS) [Member] Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Effective tax rate Effective Income Tax Rate Reconciliation, Percent Federal statutory rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Income Tax Examination, Increase (Decrease) in Liability from Prior Year Income Tax Examination, Increase (Decrease) in Liability from Prior Year Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Related to Items with Unallowable Deductions Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Related to Items with Unallowable Deductions Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Related to Items with Unallowable Deductions Income tax benefit Net investment income (expense) Expenses Operating Expenses [Abstract] Policy acquisition expenses Pre-tax (loss) income from continuing operations Net (income) loss from discontinued operations (Loss) income before equity in (losses) earnings of unconsolidated affiliates Income (Loss) from Operations before Extraordinary Items Equity in earnings of subsidiaries, net of tax Less: Net income attributable to noncontrolling interests Assets Assets [Abstract] Investments Premiums receivable Premiums Receivable, Net Reinsurance recoverable on unpaid losses Net deferred tax asset Deferred Tax Assets, Net of Valuation Allowance Other assets Other Assets Total assets Assets Liabilities Liabilities [Abstract] Ceded reinsurance payable Reinsurance Payable Other liabilities Other Liabilities Total liabilities Liabilities Allocated Share-based Compensation Expense Fixed rate CMBS Floating rate CMBS GNMA Government National Mortgage Association Certificates and Obligations (GNMA) [Member] FNMA Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] FHLMC Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] Agency commercial mortgage-backed securities Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] Residential Total mortgage-backed securities Collateralized Mortgage Backed Securities [Member] Credit card receivables Collateralized Credit Card Securities [Member] Vehicle receivables Collateralized Auto Loans [Member] Other Collateralized Securities, Other [Member] Total other asset-backed securities Asset-backed Securities, Securitized Loans and Receivables [Member] Total mortgage-backed and asset-backed securities Non-Prime Residential Mortgage Backed Security Non-Prime Residential Mortgage Backed Security [Member] Non-Prime Residential Mortgage Backed Security [Member] Average basis points of subordination Average Basis Points of Subordination Average Basis Points of Subordination Percentage of underlying loans reported as non-performing Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans Total revenues Policy acquisition expenses Disposal Group Including Discontinued Operation Underwriting Expenses Net of Tax Represents the amount of underwriting expenses, net of tax attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period. Other underwriting expenses Investments, Debt and Equity Securities [Abstract] Fixed maturity investments, at fair value Short-term investments, at amortized cost (which approximates fair value) Short-term Investments Common equity securities, at fair value Trading Securities, Equity Convertible fixed maturity investments, at fair value Other Long-term Investments Total investments Cash Net deferred tax asset Deferred Tax Assets, Net Investment income accrued Accrued Investment Income Receivable Accounts receivable on unsettled investment sales Receivables from Brokers-Dealers and Clearing Organizations Investments in subsidiaries Equity Method Investments Assets held for sale Assets Held-for-sale, at Carrying Value Accounts payable on unsettled investment purchases Payables to Broker-Dealers and Clearing Organizations Liabilities held for sale Liabilities of Assets Held-for-sale Equity [Abstract] Equity [Abstract] OneBeacon's common shareholders' equity Stockholders' Equity Attributable to Parent Total noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Total OneBeacon's common shareholders' equity and noncontrolling interests Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests Liabilities and Equity Fair Value Disclosures [Abstract] Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Statement of Cash Flows [Abstract] Cash flows from operations: Net realized and change in unrealized investment gains Net realized gain on sale of business Other operating items: Increase (Decrease) in Operating Capital [Abstract] Net change in loss and LAE reserves Net change in unearned premiums Net change in premiums receivable Net change in reinsurance recoverable on paid and unpaid losses Net change in other assets and liabilities Net maturities, purchases and sales of short-term investments Purchases of fixed maturity investments Purchases of common equity securities Purchases of convertible fixed maturity investments Contributions for other investments Net change in unsettled investment purchases and sales Net acquisitions of property and equipment Net cash provided from (used for) investing activities—continuing operations Cash dividends paid to common shareholders Payments on capital lease obligation Net cash used for financing activities—continuing operations Net increase (decrease) in cash during period Loss and loss adjustment expense (LAE) reserve activities Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Net realized investment gains Trading Securities, Realized Gain (Loss) Nature of Operations and Summary of Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Total net changes in fair value reflected in revenues Trading Securities, Change in Unrealized Holding Gain (Loss) [Abstract] Changes in net unrealized investment gains (losses) Changes in net foreign currency translation gains (losses) Foreign Currency Transaction Gain (Loss), Unrealized Total net changes in fair value reflected in revenues 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. Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Chief Executive Officer [Member] Chief Executive Officer [Member] Award Date [Axis] Award Date [Axis] Award Date [Domain] Award Date [Domain] Awards Issued in 2012 [Member] Awards Issued in 2012 [Member] Represent share-based payments awards issued in 2012. Awards Issued in 2011 [Member] Awards Issued in 2011 [Member] Represents share-based payment awards granted in 2011. Restricted Stock [Member] Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested Forfeitures (in shares) Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Unamortized grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Unamortized grant date fair value, beginning of period Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Issued 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. Unamortized grant date fair value, end of period Trading Securities Gross Unrealized Loss Gross Unrealized Gains (Losses) on Trading Securities Pretax 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 Deferred Tax Liabilities, Unrealized Gains on Trading Securities Deferred Tax Liabilities, Unrealized Gains on Trading Securities Unrealized gains (losses) on trading securities, net of tax Unrealized gains (losses) on trading securities, net of tax Unrealized gains (losses) on trading securities, net of tax Essentia Insurance Company Essentia Insurance Company [Member] Essentia Insurance Company [Member] Gain on sale of discontinued operation, pre-tax Premiums Earned, Net Investment Securities: Investments [Abstract] Convertible fixed maturity investments, at fair value Trading Securities Convertible Fixed Maturity Investments Represents the fair value of convertible fixed maturity investments that are convertible to equity securities at a future date. OneBeacon's common shareholders' equity and noncontrolling interests Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] OneBeacon's common shareholders' equity Stockholders' Equity Attributable to Parent [Abstract] Common shares and paid-in surplus (par value $0.01; authorized, 200,000,000 shares; issued and outstanding, 95,296,387 and 95,404,138 shares) Common Stocks, Including Additional Paid in Capital Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive loss, after tax Accumulated Other Comprehensive Income (Loss), Net of Tax Total OneBeacon's common shareholders' equity Total noncontrolling interests, after tax Total OneBeacon's common shareholders' equity and noncontrolling interests Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value of Financial Instruments Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Income Taxes Income Tax Disclosure [Text Block] Schedule of computation of earnings per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Long/short credit and distressed Hedge Funds Credit and Distressed [Member] Investments in registered hedge funds that invest in credit & distressed securities, taking both long and short position. Long bank loan Hedge Funds Bank Loan [Member] Investments in registered hedge funds that invest in bank loans, taking both long and short positions. Long/short equity Hedge Funds, Equity [Member] Long/short equity activist 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. Insurance Private Equity Insurance [Member] Represents investments in private equity funds of the insurance industry. Distressed residential real estate Private Equity Distressed Residential Real Estate [Member] Investments in private equity funds of distressed residential real estate. Energy infrastructure and services Private Equity Energy Infrastructure and Services [Member] Investments in private equity funds related to energy infrastructure and services. Healthcare Private Equity Healthcare [Member] Represents investments in private equity funds of the healthcare industry. Multi-sector Private Equity Multi Sector [Member] Investments in private equity funds of more than one industry. Private equity secondaries Private Equity Secondaries [Member] Investments in private equity funds of the private equity secondary market. Real estate Real Estate Funds [Member] Hedge funds and private equity funds Hedge Funds and Private Equity Funds [Member] Investments in hedge funds and private equity funds. Monthly 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. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Quarterly [Member] 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. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Frequency Annual [Member] 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. Fair Value Investments Entities That Calculate Net Asset Value Per Share Investment Redemption Frequency [Axis] 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] 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. 60 - 89 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. Investment Redemption Notice Period over 120 Days [Member] 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. Investment Redemption Notice Period 90 to 119 Days [Member] 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. Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period thirty to fifty nine Days [Member] Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period thirty to fifty nine Days [Member] For investments on which the entity calculates net asset value, the entity is required to deliver thirty to fifty nine days notice before it can redeem its investment. Fair Value Investments Entities that Calculate Net Asset Value Per Share Liquidating Investment Remaining Period [Axis] 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] 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. 1 - 3 years Liquidating Investment Remaining Period 1 to 3 Years [Member] 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. 3 - 5 years Liquidating Investment Remaining Period 3 to 5 Years [Member] 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. 5 - 10 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. >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. Other investments at fair value Trading Securities, Other Net asset value per share of unfunded commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Distributions from inactive hedge funds Distributions from Inactive Hedge Funds Distributions pending from hedge funds that are no longer active and are in process of disposing their underlying investments. Redemption requests Fair Value Investments Entities that Calculate Net Asset Value Per Share Redemption Request The amount of redemption requests submitted for investments for hedge funds.