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. Organization, Consolidation and Presentation of Financial Statements [Abstract] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Investments, Debt and Equity Securities [Abstract] 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] Fixed Maturities [Member] Fixed Maturities [Member] Equity Securities [Member] Equity Securities [Member] Industrial, Consumer And All Other [Member] Industrial, Miscellaneous, and All Others [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, 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 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 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 Other-than-temporary impairment losses recognized in net income Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities Other than temporary impairment losses recognized in earnings number of investment positions OtherThanTemporaryImpairmentLossesRecognizedInEarningsNumberOfInvestmentPositions Number of investment positions that have other-than-temporary impairment losses recognized in earnings and included in net realized investment losses. Statement of Financial Position [Abstract] Fixed maturities, amortized cost Available-for-sale Debt Securities, Amortized Cost Basis 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 Earnings Per Share [Abstract] Schedule Of Net Income Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Net Income Per Share Earnings Per Share [Text Block] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Markel Diversified Fund [Member] Markel Diversified Fund [Member] Markel Diversified Fund Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Variable Interest Entity, Consolidated, Carrying Amount, Assets Variable Interest Entity, Consolidated, Carrying Amount, Assets Variable Interest Entity, Consolidated, Carrying Amount, Liabilities Variable Interest Entity, Consolidated, Carrying Amount, Liabilities Investment in Unconsolidated Variable Interest Entity, Carrying Amount Investment in unconsolidated variable interest entity, carrying amount Investment in unconsolidated variable interest entity, carrying amount Investment in Unconsolidated Variable Interest Entity, Percent Investment in unconsolidated variable interest entity, percent Percentage ownership, investment in unconsolidated variable interest entity Variable Interest Entity, Consolidated, Carrying Amount, Note Payable Variable Interest Entity, Consolidated, Carrying Amount, Note Payable Variable Interest Entity, Consolidated, Carrying Amount, Note Payable Assets of Unconsolidated Variable Interest Entities Under Management Assets of Unconsolidated Variable Interest Entities Under Management Total assets under management related to unconsolidated variable interest entities Reinsurance Disclosures [Abstract] 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 Premiums Earned, Net Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract] Segment Reporting Disclosures Segment Reporting Disclosure [Text Block] 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 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] Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 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] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value New Accounting Pronouncements and Changes in Accounting Principles [Abstract] Recent Accounting Pronouncements New Accounting Pronouncements and Changes in Accounting Principles [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 Net Income (Loss) Attributable to Parent 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 Earnings Per Share, Basic Diluted net income per share Earnings Per Share, Diluted 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] AOCI Attributable to Parent [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 Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Balance, shares Shares, Issued Balance, redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Issuance of common stock Stock Issued During Period, Value, New Issues Issuance of common stock, shares Stock Issued During Period, Shares, New Issues Repurchase of common stock Stock Repurchased During Period, Value Repurchase of common stock, shares Stock Repurchased During Period, Shares Restricted stock units expensed Adjustments to Additional Paid in Capital, Share-based Compensation, Restricted Stock Unit or Restricted Stock Award, Requisite Service Period Recognition Restricted stock units expensed, shares Restricted Stock Expense Shares Restricted Stock Expense, Shares Adjustment of redeemable noncontrolling interests Temporary Equity, Accretion to Redemption Value Adjustment of redeemable noncontrolling interests, shares Temporary Equity Accretion To Redemption Value Shares Temporary Equity Accretion To Redemption Value, Shares Purchase of noncontrolling interest Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Purchase of noncontrolling interest, shares Adjustment To Additional Paid In Capital Purchase Of Noncontrolling Interest Shares Adjustment To Additional Paid In Capital Purchase Of Noncontrolling Interest, Shares Other Stockholders' Equity, Other Other, shares Stockholders' Equity, Other Shares Balance Balance, shares Balance, redeemable noncontrolling interests New Accounting Pronouncement [Abstract] New Accounting Pronouncement [Abstract] New Accounting Pronouncement, Change in Accounting Principle, Balance Sheet Reclassification New Accounting Pronouncement, Change in Accounting Principle, Balance Sheet Reclassification New Accounting Pronouncement, Change in Accounting Principle, Balance Sheet Reclassification, with no impact on net assets. 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] 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] Corporate Bonds [Member] All Other Corporate Bonds [Member] Total Fixed Maturities [Member] Insurance, Banks And Other Financial Institutions [Member] Banks, Trust and Insurance, Equities [Member] Total Equity Securities [Member] Short-Term Investments [Member] Short-term Investments [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] 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 Available-for-sale Securities 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] 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 Gains (losses) on securities measured at fair value through net income Gains (losses) on securities measured at fair value through net income Gains (losses) on securities measured at fair value through net income Net realized investment gains Available-for-sale Securities, Gross Realized Gain (Loss) Change in net unrealized gains on investments Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes Variable Interest Entities [Abstract] Variable Interest Entities [Abstract] Variable Interest Entities Variable Interest Entity Disclosure [Text Block] 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] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Statement, Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] ASSETS Assets [Abstract] Fixed maturities (amortized cost of $9,556,139 in 2016 and $9,038,158 in 2015) Available-for-sale Securities, Debt Securities Equity securities (cost of $2,353,941 in 2016 and $2,208,834 in 2015) Available-for-sale Securities, Equity Securities Short-term investments (estimated fair value approximates cost) Other Short-term Investments Total Investments Investments Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value 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 Goodwill Intangible assets Intangible Assets, Net (Excluding Goodwill) Other assets Other Assets Total Assets 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 $2,504,000 in 2016 and $2,403,000 in 2015) 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 Accumulated Other Comprehensive Income (Loss), Net of Tax 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 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 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 Other Comprehensive Income (Loss), Tax [Abstract] 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 Investment Gain (Loss) Attributable to Parent [Member] Net Actuarial Pension Loss [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Reclassification Out Of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Net realized investment gains, excluding other-than-temporary impairment losses Marketable Securities, Realized Gain (Loss), Excluding Other than Temporary Impairments Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest 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. Income taxes Income Tax Expense (Benefit) Net Income Other Revenues And Other Expenses Other Operating Income and Expense [Text Block] Net holding gains 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 Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Business Combinations [Abstract] 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] CapTech Ventures Inc [Member] CapTech Ventures Inc [Member] CapTech Ventures Inc [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Percentage acquired Business Acquisition, Percentage of Voting Interests Acquired Total consideration Business Combination, Consideration Transferred Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles Fair Value Measurements Fair Value Disclosures [Text Block] Loss Contingency [Abstract] Contingencies Contingencies Disclosure [Text Block] Acquisitions Business Combination Disclosure [Text Block] Subsequent Events [Abstract] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 7.35% Unsecured Senior Notes [Member] 7.35% Unsecured Senior Notes [Member] 7.35% Unsecured Senior Notes Due 2034 7.125% Unsecured Senior Notes [Member] 7.125% Unsecured Senior Notes [Member] 7.125% Unsecured Senior Notes Due 2019 7.20% Unsecured Senior Notes [Member] 7.20% Unsecured Senior Notes [Member] 7.20% Unsecured Senior Notes Due 2017 Subsequent Event [Line Items] Subsequent Event [Line Items] Subsequent Event, Date Subsequent Event, Date Debt Instrument, Face Amount Debt Instrument, Face Amount Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Maturity Date Debt Instrument, Maturity Date Proceeds from Debt, Net of Issuance Costs Proceeds from Debt, Net of Issuance Costs Debt Instrument, Repurchased Face Amount Debt Instrument, Repurchased Face Amount Early Repayment of Senior Debt Early Repayment of Senior Debt Senior Notes Senior Notes Gains (Losses) on Extinguishment of Debt Gains (Losses) on Extinguishment of Debt Effective tax rate Effective Income Tax Rate Reconciliation, Percent U.S. statutory tax rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Effective Income Tax Rate Reconciliation, Tax Credit, Percent Effective Income Tax Rate Reconciliation, Tax Credit, Percent U.S. Insurance [Member] U.S. Insurance [Member] U.S. Insurance [Member] International Insurance [Member] International Insurance [Member] International Insurance [Member] Reinsurance [Member] Reinsurance [Member] Reinsurance [Member] 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 Earned 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 profit (loss) Underwriting Profit (Loss) Earned premiums less incurred losses, loss adjustment expenses and other underwriting, acquisition and insurance expenses. Net investment income 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 Amortization of Intangible Assets Interest expense 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 Subsequent Events [Text Block] Subsequent Events [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] 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 Net Realized Investment Gains (Losses) And The Change In Net Unrealized Gains On Investments Gain (Loss) on Investments [Table 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 Other Comprehensive Income Comprehensive Income (Loss) Note [Text Block] 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] 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 Short-term investments Available-for-sale investments, estimated fair value 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 Insurance [Member] Other Insurance Product Line [Member] Manufacturing [Member] Manufacturing [Member] Manufacturing [Member] Non-Manufacturing [Member] Non-Manufacturing [Member] Non-Manufacturing [Member] Investment Management [Member] Investment Management [Member] Investment Management [Member] Other Non-Insurance [Member] Other Noninsurance Product Line [Member] Product line consisting of other types of non-insurance. Insurance Operations [Member] Insurance Operations [Member] Amount attributable to insurance operations. Markel Ventures Operations [Member] Markel Ventures Operations [Member] Amount attributable to Markel Ventures operations. Other Revenues and Other Expenses [Line Items] Other Revenues and Other Expenses [Line Items] [Line Items] for Other Revenues and Other Expenses [Table] Other revenues Other Income Other expenses Other Cost and Expense, Operating Accumulated Other Comprehensive Income [Table] Accumulated Other Comprehensive Income (Loss) [Table] Foreign Currency [Member] Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Accumulated Other Comprehensive Income [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated other comprehensive income, beginning balance 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 Reinsurance Premiums for Insurance Companies, by Product Segment [Table] Reinsurance Premiums for Insurance Companies, by Product Segment [Table] Reinsurance Policy, Type [Axis] Reinsurance Policy, Type [Axis] Reinsurance Policy [Domain] Reinsurance Policy [Domain] Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items] Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items] 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. Percentage of assumed earned premiums to net earned premiums Premiums, Percentage Assumed to Net Reinsurance recoverables Policyholder Benefits and Claims Incurred, Ceded Accounting Policies [Abstract] Basis of Accounting, Policy [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] 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 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] 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 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 Income Statement [Abstract] OPERATING REVENUES Revenues [Abstract] Other-than-temporary impairment losses Other than Temporary Impairment Losses, Investments, Available-for-sale Securities 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 Amortization of intangible assets Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Interest expense Income tax expense Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Net Income to Shareholders OTHER COMPREHENSIVE INCOME Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Net holding gains 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 Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income to Shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent NET INCOME PER SHARE Basic (in dollars per share) Diluted (in dollars per share) Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned Effects of Reinsurance [Table Text Block] 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 (used) by operating activities Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities Net Cash Provided (Used) 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 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 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 END OF PERIOD 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 Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses Reinsurance Reinsurance [Text Block]