Organization, Consolidation and Presentation of Financial Statements [Abstract] Basis Of Presentation Business Description and Basis of Presentation [Text Block] Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Statement, Equity Components, Including Redeemable Noncontrolling Interests [Axis] Equity Components, Including Redeemable Noncontrolling Interests [Axis] Information by component of equity and redeemable noncontrolling interests. Equity Component, Including Redeemable Noncontrolling Interests [Domain] Equity Component, Including Redeemable Noncontrolling Interests [Domain] Components of equity are the parts of the total Equity balance including that which is allocated to common, preferred, treasury stock, retained earnings, etc. Also includes redeemable noncontrolling interests. Total Equity [Member] Total Equity [Member] Components of equity are the parts of the Total Equity balance. Total Shareholders' Equity [Member] Parent [Member] Common Stock [Member] Common Stock [Member] Retained Earnings [Member] Retained Earnings [Member] Accumulated Other Comprehensive Income [Member] Accumulated Other Comprehensive Income (Loss) [Member] Noncontrolling Interests [Member] Noncontrolling Interest [Member] Redeemable Noncontrolling Interests [Member] Redeemable Noncontrolling Interests, Temporary Equity [Member] The noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. Statement [Line Items] Statement [Line Items] Balance, redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Net income (loss) Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Comprehensive Income (Loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Issuance of common stock Stock Issued During Period, Value, New Issues Repurchase of common stock Stock Repurchased During Period, Value Restricted stock units expensed Adjustments to Additional Paid in Capital, Share-based Compensation, Restricted Stock Unit or Restricted Stock Award, Requisite Service Period Recognition Acquisition of Alterra Stock Issued During Period, Value, Acquisitions Adjustment of redeemable noncontrolling interests Temporary Equity, Accretion to Redemption Value Purchase of noncontrolling interest Adjustment To Additional Paid In Capital Purchase of Noncontrolling Interests Amount of other increase (decrease) in additional paid in capital (APIC) attributable to the purchase of noncontrolling interests during the period. This amount represents the different between the consideration transferred to purchase the noncontrolling interest and the carrying value of the noncontrolling interest. Purchase of noncontrolling interest Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Purchase of noncontrolling interest Change in Equity Purchase of Noncontrolling Interests Net change in total equity attributable to the purchase of noncontrolling interests during the period. This amount represents the parent’s adjustment to APIC for the difference between the consideration transferred to purchase the noncontrolling interest and the carrying value of the noncontrolling interest and the decrease in noncontrolling interests during the period. Other Stockholders' Equity, Other Balance Balance, redeemable noncontrolling interests Earnings Per Share [Abstract] Schedule Of Net Income Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Investments, Debt and Equity Securities [Abstract] Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Taxable Municipal Bonds [Member] Taxable Municipal Bonds [Member] Other Taxable Bonds [Member] Other Taxable Bonds [Member] Other Taxable Bonds [Member] Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Municipal bonds (tax-exempt) Interest Income, Securities, Operating, Tax Exempt Taxable bonds Interest Income, Securities, Operating, Taxable Short-term investments, including overnight deposits Interest Income, Deposits with Financial Institutions Dividends on equity securities Investment Income, Dividend Change in fair value of credit default swap Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net Income from equity method investments Income (Loss) from Equity Method Investments Other Other Interest and Dividend Income Total investment income Investment Income, Interest and Dividend Investment expenses Investment Income, Investment Expense Net investment income Net Investment Income Fair Value Disclosures [Abstract] Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Summary Of Changes In Level 3 Liabilities Measured At Fair Value On A Recurring Basis Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Other Income and Expenses [Abstract] Summary Of Other Revenues And Other Expenses By Component Schedule Of Other Revenues And Other Expenses By Component [Table Text Block] Tabular disclosure of each detailed component of other revenues and other expenses. Goodwill [Abstract] Goodwill [Abstract] Components Of Goodwill Schedule of Goodwill [Table Text Block] Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Statement, Operating Activities Segment [Axis] Operating Activities [Axis] Segment, Operating Activities [Domain] Operating Activities [Domain] Segment Assets [Member] Continuing Operations [Member] Statement, Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Investing [Member] Investing Member Investing [Member] Underwriting [Member] Underwriting Segment Member Underwriting Segment [Member] Non-Insurance Operations [Member] Non-Insurance Operations [Member] Amount attributable to non-insurance operations. Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Assets Assets New Accounting Pronouncements and Changes in Accounting Principles [Abstract] Recent Accounting Pronouncements New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Commercial Mortgage-Backed Securities [Member] Commercial Mortgage Backed Securities [Member] Residential Mortgage-Backed Securities [Member] Residential Mortgage Backed Securities [Member] Asset-Backed Securities [Member] Asset-backed Securities [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Due in one year or less, amortized cost Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis Due after one year through five years, amortized cost Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis Due after five years through ten years, amortized cost Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis Due after ten years, amortized cost Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis Amortized cost, sub-total Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Fixed maturities, amortized cost Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis Total fixed maturities, amortized cost Available-for-sale Debt Securities, Amortized Cost Basis Due in one year or less, estimated fair value Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Due after one year through five years, estimated fair value Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Due after five years through ten years, estimated fair value Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value Due after ten years, estimated fair value Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Estimated fair value, sub-total Available-for-sale Securities, Debt Maturities, Single Maturity Date Fixed maturities, estimated fair value Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value Total fixed maturities, estimated fair value Available-for-sale Securities, Debt Securities Derivatives, beginning of period Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Total gains included in net income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Total gains included in other comprehensive income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) Transfers into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 Derivatives, end of period Net unrealized gains included in net income relating to liabilities held at June 30, 2014 and 2013 Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) Debt Disclosure [Abstract] Senior Long-Term Debt And Other Debt Debt Disclosure [Text Block] Income Statement [Abstract] Statement, Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] OPERATING REVENUES Revenues [Abstract] Earned premiums Premiums Earned, Net Net investment income Other-than-temporary impairment losses Other than Temporary Impairment Losses, Investments, Available-for-sale Securities Net realized investment gains, excluding other-than-temporary impairment losses Marketable Securities, Realized Gain (Loss), Excluding Other than Temporary Impairments Net realized investment gains Available-for-sale Securities, Gross Realized Gain (Loss) Other revenues Other Income Total Operating Revenues Revenues OPERATING EXPENSES Operating Expenses [Abstract] Losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net, Property, Casualty and Liability Underwriting, acquisition and insurance expenses Underwriting, acquisition and insurance expenses Costs incurred during the period related to selling the Company's products and services including the amortization of deferred policy acquisition costs charged to expense in the period, as well as other costs incurred during the period, such as those relating to general administration and policy maintenance. Amortization of intangible assets Amortization of Intangible Assets Other expenses Other Cost and Expense, Operating Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Interest expense Interest Expense Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax expense Income Tax Expense (Benefit) Net Income Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Net Income to Shareholders Net Income (Loss) Attributable to Parent OTHER COMPREHENSIVE INCOME (LOSS) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Net holding gains (losses) arising during the period Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities Reclassification adjustments for net gains included in net income Reclassification adjustments for net gains (losses) included in net income (loss) Reclassification adjustment from Other Comprehensive Income (Loss) to net income (loss), which includes net unrealized gains or losses to net realized gains or losses, which includes reclassifications due to write-downs, sales and other items, net of tax. Change in net unrealized gains on investments, net of taxes Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax Change in foreign currency translation adjustments, net of taxes Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Change in net actuarial pension loss, net of taxes Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income (Loss) Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income (Loss) to Shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent NET INCOME PER SHARE Basic (in dollars per share) Earnings Per Share, Basic Diluted (in dollars per share) Earnings Per Share, Diluted Reinsurance Disclosures [Abstract] Reinsurance Reinsurance [Text Block] Other Comprehensive Income (Loss), Tax [Abstract] Change In Accumulated Other Comprehensive Income By Component, Net Of Taxes And Noncontrolling Interest Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Summary Of Tax Expense (Benefit) Of Other Comprehensive Income (Loss) Schedule Of Tax Expense Benefit Of Other Comprehensive Income [Table Text Block] Schedule of tax expense (benefit) associated with each component of other comprehensive income. Reclassifications From Accumulated Other Comprehensive Income Into Income, By Component Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Available-For-Sale Investments Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position Schedule of Unrealized Loss on Investments [Table Text Block] Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity Date Investments Classified by Contractual Maturity Date [Table Text Block] Components Of Net Investment Income Investment Income [Table Text Block] Summary Of Activity For Credit Losses Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table Text Block] Summary Of Net Realized Investment Gains And The Change In Net Unrealized Gains On Investments Gain (Loss) on Investments [Table Text Block] Summary Of Other-Than-Temporary Impairment Losses Recognized In Net Income And In Net Realized Investment Gains By Investment Type Schedule Of Other Than Temporary Impairment Losses By Investment Type [Table Text Block] The tabular disclosure of the amount of other-than-temporary impairment losses recognized in earnings and included in net realized investment gains (losses), by investment type. Business Combinations [Abstract] Acquisitions Business Combination Disclosure [Text Block] Statement of Financial Position [Abstract] Fixed maturities, amortized cost Equity securities, cost Available-for-sale Equity Securities, Amortized Cost Basis Senior long-term debt and other debt, estimated fair value Debt Instrument, Fair Value Disclosure Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 6.25% Unsecured Senior Notes [Member] Six Point Two Five Percent Unsecured Senior Notes [Member] Six Point Two Five Percent Unsecured Senior Notes [Member] 7.20% Unsecured Senior Notes [Member] Seven Point Two Zero Percent Unsecured Senior Notes [Member] Seven Point Two Zero Percent Unsecured Senior Notes [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Senior Credit Facility [Member] Senior Credit Facility [Member] Senior Credit Facility [Member] Short-term Debt, Type [Axis] Short-term Debt, Type [Axis] Short-term Debt, Type [Domain] Short-term Debt, Type [Domain] Secured Letters Of Credit [Member] Secured Letters Of Credit [Member] Secured Letters Of Credit [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Debt instrument, face amount Debt Instrument, Face Amount Debt instrument, interest rate Debt Instrument, Interest Rate, Stated Percentage Line of credit facility, capacity available Line of Credit Facility, Maximum Borrowing Capacity Line of credit facility, potential increased maximum capacity LineOfCreditFacilityBorrowingCapacityIncrease Potential increased maximum capacity, under the credit facility, without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. Previous line of credit facility, borrowing capacity PreviousLineOfCreditFacilityMaximumBorrowingCapacity Maximum borrowing capacity under the previous credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. Schedule of Gain (Loss) on Investments [Table] Schedule of Gain (Loss) on Investments [Table] Sales Of Fixed Maturities [Member] Sales Of Fixed Maturities Member Sales Of Fixed Maturities [Member] Sales Of Equity Securities [Member] Sales Of Equity Securities Member Sales Of Equity Securities [Member] Other [Member] Other Debt Obligations [Member] Fixed Maturities [Member] Fixed Maturities [Member] Equity Securities [Member] Equity Securities [Member] Short-Term Investments [Member] Short-term Investments [Member] Gain (Loss) on Investments [Line Items] Gain (Loss) on Investments [Line Items] Realized gains Available-for-sale Securities, Gross Realized Gains Realized losses Available-for-sale Securities, Gross Realized Losses Other-than-temporary impairment losses recognized in net income Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities Change in net unrealized gains on investments Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes Statement of Cash Flows [Abstract] OPERATING ACTIVITIES Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Adjustments to reconcile net income to net cash provided by operating activities Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities Net Cash Provided By Operating Activities Net Cash Provided by (Used in) Operating Activities INVESTING ACTIVITIES Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from sales of fixed maturities and equity securities Proceeds from Sale of Available-for-sale Securities Proceeds from maturities, calls and prepayments of fixed maturities Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities Cost of fixed maturities and equity securities purchased Payments to Acquire Available-for-sale Securities Net change in short-term investments Payments for (Proceeds from) Short-term Investments Proceeds from sales of equity method investments Proceeds from Sale of Equity Method Investments Cost of equity method investments Payments to Acquire Equity Method Investments Change in restricted cash and cash equivalents Increase (Decrease) in Restricted Cash Additions to property and equipment Payments to Acquire Property, Plant, and Equipment Acquisitions, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired Other Payments for (Proceeds from) Other Investing Activities Net Cash Provided (Used) By Investing Activities Net Cash Provided by (Used in) Investing Activities FINANCING ACTIVITIES Net Cash Provided by (Used in) Financing Activities [Abstract] Additions to senior long-term debt and other debt Proceeds from Issuance of Debt Repayment of senior long-term debt and other debt Repayments of Debt and Capital Lease Obligations Repurchases of common stock Payments for Repurchase of Common Stock Issuance of common stock Proceeds from Issuance of Common Stock Purchase of noncontrolling interests Payments to Acquire Additional Interest in Subsidiaries Distributions to noncontrolling interests Payments of Ordinary Dividends, Noncontrolling Interest Other Proceeds from (Payments for) Other Financing Activities Net Cash Provided (Used) By Financing Activities Net Cash Provided by (Used in) Financing Activities Effect of foreign currency rate changes on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents Increase (decrease) in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of period Cash and Cash Equivalents, at Carrying Value CASH AND CASH EQUIVALENTS AT END OF PERIOD Net holding gains (losses) arising during the period Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income Loss, Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities Reclassification adjustments for net gains included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Change in net unrealized gains on investments Other Comprehensive Income (Loss), Available-for-sale Securities, Tax Change in foreign currency translation adjustments Other Comprehensive Income (Loss), Foreign Currency Translation Gain (Loss) Arising During Period, Tax Change in net actuarial pension loss Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax Total Other Comprehensive Income (Loss), Tax Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] Cumulative credit loss, beginning balance Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held Other-than-temporary impairment losses not previously recognized Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment Increases related to other-than-temporary impairment losses previously recognized Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, Additional Credit Losses Total additions Total Additions Of Other-Than-Temporary Impairment Credit Losses Recognized In Earnings Total additions of other-than-temporary impairment credit losses recognized in earnings. Sales or maturities of fixed maturities on which credit losses were recognized Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold Cumulative credit loss, ending balance Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Other Revenues and Other Expenses [Table] Other Revenues and Other Expenses [Table] Other Revenues and Other Expenses [Table] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Managing General Agent Operations [Member] Managing General Agent Operations [Member] Managing General Agent Operations [Member] Life And Annuity [Member] Life and Annuity Insurance Product Line [Member] Other [Member] Other Insurance Product Line [Member] Manufacturing [Member] Manufacturing [Member] Manufacturing [Member] Non-Manufacturing [Member] Non-Manufacturing [Member] Non-Manufacturing [Member] Consulting Services [Member] Consulting Services [Member] Consulting Services [Member] Insurance Operations [Member] Insurance Operations [Member] Amount attributable to insurance operations. Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Markel Ventures Operations [Member] Markel Ventures Operations [Member] Amount attributable to Markel Ventures operations. Abbey Protection plc [Member] Abbey Protection plc [Member] Abbey Protection plc [Member] Other Revenues and Other Expenses [Line Items] Other Revenues and Other Expenses [Line Items] [Line Items] for Other Revenues and Other Expenses [Table] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition [Domain] Business Acquisition, Acquiree [Domain] May 1, 2013 Alterra Capital Holdings [Member] Alterra Capital Holdings [Member] Alterra Capital Holdings [Member] January 17, 2014 Abbey Protection plc [Member] July 23, 2014 Cottrell Inc [Member] Cottrell Inc [Member] Cottrell Inc [Member] U.S. Insurance Segment [Member] U.S. Insurance [Member] U.S. Insurance [Member] International Insurance Segment [Member] International Insurance [Member] International Insurance [Member] Reinsurance Segment [Member] Reinsurance [Member] Reinsurance [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Customer Relationships [Member] Customer Relationships [Member] Trade Names [Member] Trade Names [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Percentage acquired Business Acquisition, Percentage of Voting Interests Acquired Number of Markel Corporation shares issued for each outstanding share of Alterra common stock Business Combinations Rights Received Per Current Shares Outstanding Shares Rights received for each issued and outstanding share of Alterra common stock, shares of Markel Corporation common stock. Cash paid for each outstanding share of Alterra common stock Business Combination Rights Received Per Current Shares Outstanding Per Share Cash Rights received for each issued and outstanding share of Alterra common stock, cash. Total purchase consideration Business Combination, Consideration Transferred Cash consideration Payments to Acquire Businesses, Gross Stock consideration, shares Business Acquisition, Equity Interest Issued or Issuable, Number of Shares Goodwill recognized Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Goodwill Amount as of the acquisition date which is the cumulative amount paid and (if applicable) the fair value of any noncontrolling interest in the acquiree, in excess of the fair value of net assets acquired in one or more business combination transactions. Goodwill, expected tax deductible Business Acquisition, Goodwill, Expected Tax Deductible Amount Indefinite lived intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets Other intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Other intangible assets, weighted average amortization period, years Acquired Finite-lived Intangible Assets, Weighted Average Useful Life Premiums under management Premiums Under Management Amount of premiums under management. Segment Reporting Disclosures Segment Reporting Disclosure [Text Block] Document And Entity Information [Abstract] Document and Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key 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 Schedule of Goodwill [Table] Schedule of Goodwill [Table] U.S. Insurance [Member] International Insurance [Member] Reinsurance [Member] Other [Member] Goodwill [Line Items] Goodwill [Line Items] Goodwill, beginning balance Goodwill Acquisitions Goodwill, Acquired During Period Foreign currency movements and other adjustments Goodwill, Translation Adjustments Goodwill, ending balance Direct premiums written Direct Premiums Written Direct premiums earned Direct Premiums Earned Assumed premiums written Assumed Premiums Written Assumed premiums earned Assumed Premiums Earned Ceded premiums written Ceded Premiums Written Ceded premiums earned Ceded Premiums Earned Net premiums written Premiums Written, Net Net premiums earned Credit default swap, notional value Derivative, Notional Amount Fair value, level 1 to level 2 transfers, amount Fair Value, Assets And Liabilities, Level 1 To Level 2 Transfers, Amount Amount of transfers of assets and liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. Fair value, level 2 to level 1 transfers, amount Fair Value, Assets And Liabilities, Level 2 To Level 1, Transfers Amount Amount of transfers of assets and liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. Assets measured at fair value on a nonrecurring basis Assets, Fair Value Disclosure, Nonrecurring Liabilities measured at fair value on a nonrecurring basis Liabilities, Fair Value Disclosure, Nonrecurring Reclassification Out Of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification Out Of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification Out Of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification Out Of Accumulated Other Comprehensive Income [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Equity Components [Axis] Equity Components [Axis] Total [Domain] Equity Component [Domain] Unrealized Holding Gains On Available-For-Sale Securities [Member] Accumulated Net Unrealized Investment Gain (Loss) [Member] Net Actuarial Pension Loss [Member] Accumulated Defined Benefit Plans Adjustment [Member] Reclassification Out Of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Other-than-temporary impairment losses recognized in net income Underwriting, acquisition and insurance expenses Income taxes Accumulated Other Comprehensive Income [Table] Accumulated Other Comprehensive Income (Loss) [Table] Foreign Currency [Member] Accumulated Translation Adjustment [Member] Accumulated Other Comprehensive Income [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated other comprehensive income, beginning balance Accumulated Other Comprehensive Income (Loss), Net of Tax Other comprehensive income (loss) before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from accumulated other comprehensive income Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Total other comprehensive income (loss) Other Comprehensive Income Loss Net Of Tax And Noncontrolling Interest Amount of other comprehensive income (loss) attributable to parent entity, net of tax and noncontrolling interest. Accumulated other comprehensive income, ending balance Goodwill Goodwill Disclosure [Text Block] Schedule of Earnings Per Share [Table] Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Stock Options [Member] Employee Stock Option [Member] Restricted Stock Units (RSUs) [Member] Restricted Stock Units (RSUs) [Member] Earnings Per Share [Line Items] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Net income to shareholders Adjustment of redeemable noncontrolling interests Temporary Equity, Accretion to Redemption Value, Adjustment Adjusted net income to shareholders Adjusted Net Income (Loss) Net income (loss) after adjustment for temporary equity accretion to redemption value. Basic common shares outstanding Weighted Average Number of Shares Outstanding, Basic Dilutive potential common shares Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Diluted shares outstanding Weighted Average Number of Shares Outstanding, Diluted Basic net income per share Diluted net income per share Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value by Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring [Member] Fair Value, Measurements, Recurring [Member] U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] US Treasury and Government [Member] Obligations Of States, Municipalities And Political Subdivisions [Member] US States and Political Subdivisions Debt Securities [Member] Foreign Governments [Member] Foreign Government Debt Securities [Member] Corporate Bonds [Member] All Other Corporate Bonds [Member] Insurance, Banks And Other Financial Institutions [Member] Banks, Trust and Insurance, Equities [Member] Industrial, Consumer And All Other [Member] Industrial, Miscellaneous, and All Others [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Total fixed maturities Total equity securities Available-for-sale Securities, Equity Securities Short-term investments Other Short-term Investments Available-for-sale investments, estimated fair value Available-for-sale Securities Derivative contracts Credit Risk Derivative Liabilities, at Fair Value Fair Value Measurements Fair Value Disclosures [Text Block] Company's Segment Disclosures Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block] Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets Reconciliation of Assets from Segment to Consolidated [Table Text Block] Percentage of issued and outstanding common stock acquired Total Fixed Maturities [Member] Total Equity Securities [Member] Available-for-sale securities, estimated fair value, less than 12 months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Available-for-sale securities, estimated fair value, 12 months or longer Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Available-for-sale securities, estimated fair value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Alterra Capital Holdings [Member] Schedule of Investments [Line Items] Schedule of Investments [Line Items] Available-for-sale securities in unrealized loss position, number of positions Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Available-for-sale securities, estimated fair value Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Effect Of Reinsurance On Premiums Written And Earned Effects of Reinsurance [Table Text Block] Percentage of ceded earned premiums to gross earned premiums Premiums Percentage Ceded To Gross Percentage of ceded premiums earned by the entity to gross premiums earned. Reinsurance recoverables Policyholder Benefits and Claims Incurred, Ceded Other Revenues And Other Expenses Other Operating Income and Expense [Text Block] Other Insurance (Discontinued Lines) [Member] Other Insurance Discontinued Lines Member Other Insurance (Discontinued Lines) [Member] Number of reportable ongoing underwriting segments Number Of Reportable Ongoing Underwriting Segments The number of segments reported by the entity through which our ongoing underwriting operations are conducted. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. Gross premium volume Premiums Written, Gross Net written premiums Losses and loss adjustment expenses, current accident year Incurred Claims Property Casualty and Liability Current Year Provision for benefits, claims and claims settlement expenses incurred during the reporting period related to the current accident year for property, casualty and liability insurance net of the effects of contracts assumed and ceded. Losses and loss adjustment expenses, prior accident years Incurred Claims Property Casualty and Liability Prior Years Provision for benefits, claims and claims settlement expenses incurred related to prior accident years for property, casualty and liability insurance net of the effects of contracts assumed and ceded. Underwriting, acquisition and insurance expenses Transaction costs and other acquisition-related expenses Underwriting Acquisition And Insurance Expenses Transaction Costs And Other Acquisition Related Expenses Costs incurred during the period related to transaction costs and other acquisition-related costs in connection with the acquisition of Alterra. Such costs include but are not limited to: finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; severance costs; stay bonuses; other compensation costs related to the accelerated payment of long-term incentive compensation awards and restricted stock awards; and may include costs of registering and issuing debt and equity securities. Transaction costs Business Combination, Acquisition Related Costs Other acquisition related costs, severance costs Business Combination Severance Costs Amount of severance costs incurred during the period for current employees terminated in connection with the acquisition of Alterra. Other acquisition related costs, stay bonuses Business Combination Stay Bonus Costs Amount of costs related to bonuses to be provided to current employees assuming they remain employed with the Company one year from the acquisition date. Other acquisition related costs, other compensation costs Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost All other expenses Underwriting Acquisition And Insurance Expenses All Other Expenses Costs incurred during the period related to selling the Company's products and services including the amortization of deferred policy acquisition costs charged to expense in the period, as well as other costs incurred during the period, such as those relating to general administration and policy maintenance. Amount excludes expense representing the transaction costs and other acquisition-related expenses incurred in connection with the acquisition of Alterra. Underwriting profit (loss) Underwriting Profit (Loss) Earned premiums less incurred losses, loss adjustment expenses and other underwriting, acquisition and insurance expenses. Net realized investment gains Other revenues (insurance) Other Income, Insurance Reflects the sum of all other revenue and income recognized related to the entity's insurance operations. Other expenses (insurance) Other Operating Costs and Expense, Insurance The total amount of other operating costs and expense items that are associated with the entity's insurance operations. Segment profit (loss) Segment Profit (Loss) Amount of profit or loss recognized by the reportable segment. Other revenues (non-insurance) Other Income, Non-Insurance Reflects the sum of all other revenue and income recognized related to the entity's non-insurance operations. Other expenses (non-insurance) Other Operating Costs and Expense, Non-Insurance The total amount of other operating costs and expense items that are associated with the entity's non-insurance operations. Amortization of intangible assets Interest expense U.S. GAAP combined ratio Combined Ratio U.S. GAAP combined ratio - not meaniningful US GAAP combined ratio - not meaniningful US GAAP combined ratio - not meaniningful Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Summary of Investments, Other than Investments in Related Parties, by Type [Axis] Summary of Investments, Other than Investments in Related Parties, by Type [Axis] Summary of Investments, by Type [Domain] Summary of Investments, by Type [Domain] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Loss Contingency [Abstract] Contingencies Contingencies Disclosure [Text Block] Net Income Per Share Earnings Per Share [Text Block] ASSETS Assets [Abstract] Fixed maturities (amortized cost of $9,983,583 in 2014 and $10,129,141 in 2013) Equity securities (cost of $1,762,064 in 2014 and $1,566,553 in 2013) Short-term investments (estimated fair value approximates cost) Total Investments Investments Cash and cash equivalents Restricted cash and cash equivalents Restricted Cash and Cash Equivalents Receivables Premiums and Other Receivables, Net Reinsurance recoverable on unpaid losses Reinsurance Recoverables, Incurred but Not Reported Claims Reinsurance recoverable on paid losses Reinsurance Receivables Deferred policy acquisition costs Deferred Policy Acquisition Costs Prepaid reinsurance premiums Prepaid Reinsurance Premiums Goodwill Intangible assets Intangible Assets, Net (Excluding Goodwill) Other assets Other Assets Total Assets LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Unpaid losses and loss adjustment expenses Liability for Claims and Claims Adjustment Expense Life and annuity benefits Liability for Future Policy Benefits Unearned premiums Unearned Premiums Payables to insurance and reinsurance companies Reinsurance Payable Senior long-term debt and other debt (estimated fair value of $2,484,000 in 2014 and $2,372,000 in 2013) Debt and Capital Lease Obligations Other liabilities Other Liabilities Total Liabilities Liabilities Redeemable noncontrolling interests Commitments and contingencies Commitments and Contingencies Shareholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Common stock Common Stock, Value, Issued Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income Total Shareholders' Equity Stockholders' Equity Attributable to Parent Noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Total Equity Total Liabilities and Equity Liabilities and Equity Available-for-sale investments, amortized cost Available-for-sale Securities, Amortized Cost Basis Available-for-sale investments, gross unrealized holding gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Available-for-sale investments, gross unrealized holding losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale investments, unrealized other-than-temporary impairment losses Available-For-Sale Investments, Unrealized Other-Than-Temporary Impairment Losses This item represents the gross unrealized other-than-temporary impairment losses for debt securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities. Available-for-sale investments, estimated fair value