Disclosure Investment Information [Abstract] -- None. No documentation exists for this element. -- Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Fixed maturities and fixed maturities pledged under securities lending agreements Fixed Maturities [Member] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Mortgage backed securities Residential Mortgage Backed Securities [Member] Commercial mortgage backed securities Commercial Mortgage Backed Securities [Member] Asset backed securities Asset-backed Securities [Member] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Estimated Fair Value: Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Due after one year through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Due after five years through 10 years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Due after 10 years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Single maturity date Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value Securities without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Estimated fair value Debt Securities, Available-for-sale Amortized Cost: Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost Due after one year through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost Due after five years through 10 years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost Due after 10 years Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost Single maturity date Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Securities without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Amortized cost Debt Securities, Available-for-sale, Amortized Cost Reinsurance Disclosures [Abstract] Effects of reinsurance Effects of Reinsurance [Table Text Block] Summary of reinsurance recoverables on paid and unpaid losses Ceded Credit Risk [Table Text Block] Coverage and retention Reinsurance Coverage And Retention [Table Text Block] Summary of reinsurance coverage and retention. Debt Disclosure [Abstract] Schedule of senior notes Schedule of Long-term Debt Instruments [Table Text Block] Schedule of outstanding revolving credit agreement borrowings Schedule of Line of Credit Facilities [Table Text Block] Stockholders' Equity Note [Abstract] Share split conversion ratio Stockholders' Equity Note, Stock Split, Conversion Ratio Number of common shares authorized Common Stock, Shares Authorized Common shares, par value per share Common Stock, Par or Stated Value Per Share Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Earnings Per Share [Abstract] Numerator: Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net (income) loss attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Net income available to Arch Net Income (Loss) Attributable to Parent Preferred dividends Preferred Stock Dividends, Income Statement Impact Loss on redemption of preferred shares Preferred Stock Redemption Premium Net income available to Arch common shareholders Net Income (Loss) Available to Common Stockholders, Basic Denominator: Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract] Weighted average common shares outstanding Weighted Average Number of Shares Issued, Basic Series D preferred securities Weighted Average Number of Shares, Contingently Issuable Weighted average common shares outstanding — basic Weighted Average Number of Shares Outstanding, Basic Effect of dilutive common share equivalents: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Nonvested restricted shares Incremental Common Shares Attributable to Dilutive Effect of Nonvested Shares with Forfeitable Dividends Stock options Incremental Common Shares Attributable to Dilutive Effect of Call Options and Warrants Weighted average common shares and common share equivalents outstanding — diluted Weighted Average Number of Shares Outstanding, Diluted Earnings per common share: Earnings Per Share, Basic and Diluted [Abstract] Basic (per share) Earnings Per Share, Basic Diluted (per share) Earnings Per Share, Diluted Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Series D Preferred Shares Series D Preferred Stock [Member] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Stock Options Share-based Payment Arrangement, Option [Member] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive securities excluded from computation of earnings per common share (shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Fixed maturities Short-term investments Short-term Investments [Member] Equity securities Equity Securities [Member] Other investments Other Investments [Member] Corporate bonds Corporate Debt Securities [Member] Non-US government securities Non-US government securities [Member] This category includes information about debt securities (such as bonds) issued by a national, local, or municipal government not within the nation of domicile of the reporting entity. US government and government agencies US Treasury and Government [Member] Municipal bonds Municipal Bonds [Member] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Net impairment losses recognized in earnings Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net Disclosure Statutory Information [Abstract] Disclosure Statutory Information [Abstract] Statutory Accounting Practices [Table] Statutory Accounting Practices [Table] Jurisdiction [Axis] Jurisdiction [Axis] Statutory Accounting Practices, Jurisdiction [Domain] Statutory Accounting Practices, Jurisdiction [Domain] Bermuda BERMUDA Ireland IRELAND United States UNITED STATES United Kingdom UNITED KINGDOM Canada CANADA Statutory Accounting Practices [Line Items] Statutory Accounting Practices [Line Items] Actual statutory capital and surplus Statutory Accounting Practices, Statutory Capital and Surplus, Balance Required statutory capital and surplus Statutory Accounting Practices, Statutory Capital and Surplus Required Statutory net income (loss) Statutory Accounting Practices, Statutory Net Income Amount Accounting Policies [Abstract] Significant accounting policies Basis of Presentation and Significant Accounting Policies [Text Block] Income Statement [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario [Domain] Scenario [Domain] Statement [Line Items] Statement [Line Items] Revenues Revenues [Abstract] Net premiums written Premiums Written, Net Change in unearned premiums (Increase) Decrease in Unearned Premiums, Net Net premiums earned Premiums Earned, Net Net investment income Investment Income, Net Net realized gains (losses) Marketable Securities, Gain (Loss), Excluding Other-than-temporary Impairment Loss Other underwriting income Insurance Commissions and Fees Equity in net income of investments accounted for using the equity method Income (Loss) from Equity Method Investments Other income (loss) Other Operating Income (Expense), Net Total revenues Revenues Expenses Benefits, Losses and Expenses [Abstract] Losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Acquisition expenses Deferred Policy Acquisition Costs, Amortization Expense Other operating expenses Other Cost and Expense, Operating Corporate expenses General and Administrative Expense Amortization of intangible assets Amortization of Intangible Assets Interest expense Interest Expense Net foreign exchange losses (gains) Foreign Currency Transaction Gain (Loss), before Tax Total expenses Benefits, Losses and Expenses Income (loss) before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Current tax expense (benefit) Current Income Tax Expense (Benefit) Deferred tax expense (benefit) Deferred Income Tax Expense (Benefit) Income tax expense Income Tax Expense (Benefit) Net income Net income available to Arch Net income available to Arch common shareholders Net income per common share and common share equivalent Weighted average common shares and common share equivalents outstanding Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic (shares) Diluted (shares) Schedule Of Other Investments [Table] Schedule Of Other Investments [Table] -- None. No documentation exists for this element. -- Available for sale Available-for-sale Securities [Member] Fair value option Fair Value Option [Member] Investments accounted for using the fair value option. Investment Secondary Categorization [Axis] Investment Secondary Categorization [Axis] Investments by Secondary Categorization [Domain] Investments by Secondary Categorization [Domain] Term loan investments Term Loan Investments [Member] -- None. No documentation exists for this element. -- Lending Other Investments Lending [Member] Other investments characterized as lending. Credit related funds Credit Related Funds [Member] Credit Related Funds [Member] Energy Other Investments Energy [Member] Other investments characterized as energy. Investment grade fixed income Investment Grade Fixed Income [Member] -- None. No documentation exists for this element. -- Infrastructure Other Investments Infrastructure [Member] Other Investments Infrastructure [Member] Private equity Private Equity Funds [Member] Real estate Real Estate Funds [Member] Schedule Of Other Investments [Line Items] Schedule Of Other Investments [Line Items] -- None. No documentation exists for this element. -- Other investments and fair value option investments Other Investments And Other Fair Value Option Investments Other investments and fair value option investments. Other investments par Other investments par Par value of term loan investments held at fair value under the fair value option afforded by accounting guidance regarding the fair value option for financial assets and liabilities Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Goodwill And Intangible Assets [Table] Schedule of Goodwill And Intangible Assets [Table] Schedule of Goodwill And Intangible Assets [Table] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Goodwill Goodwill [Member] Intangible assets with an indefinite life Indefinite-lived Intangible Assets [Member] Intangible assets with a finite life Finite-Lived Intangible Assets [Member] Schedule of Goodwill And Intangible Assets [Line Items] Schedule of Goodwill And Intangible Assets [Line Items] [Line Items] for Schedule of Goodwill And Intangible Assets [Table] Goodwill Goodwill [Roll Forward] Goodwill, beginning of year Goodwill Goodwill, acquisitions Goodwill, Acquired During Period Goodwill, impairment Goodwill, Impairment Loss Goodwill, foreign currency translation adjustment Goodwill, Foreign Currency Translation Gain (Loss) Goodwill, end of year Intangible assets (indefinite life) Indefinite-lived Intangible Assets (Excluding Goodwill) [Abstract] Intangible assets with an indefinite life, beginning of year Indefinite-lived Intangible Assets (Excluding Goodwill) Intangible assets with an indefinite life, acquisitions Indefinite-lived Intangible Assets Acquired Intangible assets with an indefinite life, impairment Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) Intangible assets with an indefinite life, foreign currency translation adjustment Indefinite-lived Intangible Assets, Foreign Currency Translation Gain (Loss) Intangible assets with an indefinite life, end of year Intangible assets (finite life) Finite-lived Intangible Assets [Roll Forward] Intangible assets with a finite life, beginning of year Finite-Lived Intangible Assets, Net Intangible assets with a finite life, acquisitions Finite-lived Intangible Assets Acquired Intangible assets with a finite life, amortization Intangible assets with a finite life, foreign currency translation adjustment Finite Lived Intangible Assets, Foreign Currency Translation Gain (Loss) Intangible assets with a finite life, impairment Impairment of Intangible Assets, Finite-lived Intangible assets with a finite life, end of year Total Intangible Assets, Net (Including Goodwill) [Abstract] Goodwill and intangible assets, beginning of year Intangible Assets, Net (Including Goodwill) Goodwill and intangible assets, acquisitions Intangible Assets Including Goodwill Acquired During the Period Intangible Assets Including Goodwill Acquired During the Period Goodwill and intangible assets impairment Goodwill and Intangible Asset Impairment Goodwill and intangible assets, foreign currency movements and other adjustments Intangible Assets Including Goodwill, Foreign Currency Translation And Other Adjustments Intangible Assets Including Goodwill, Foreign Currency Translation And Other Adjustments Goodwill and intangible assets, end of year Goodwill, gross balance Goodwill, Gross Indefinite-lived intangible assets, gross balance Indefinite-lived Intangible Assets (Excluding Goodwill) Gross Amount Amount of assets, before impairment, lacking physical substance and having a projected indefinite period of benefit. Excludes financial assets and goodwill. Intangible assets with a finite life, gross balance Finite-Lived Intangible Assets, Gross Goodwill and intangible assets, gross balance Intangible Assets, Gross (Including Goodwill) Amount before accumulated amortization of intangible assets and translation adjustments, including goodwill. Intangible assets with a finite life, accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Goodwill and intangible assets, accumulated foreign currency translation and other adjustments Intangible Assets Including Goodwill, Accumulated Foreign Currency And Other Adjustments Intangible Assets Including Goodwill, Accumulated Foreign Currency And Other Adjustments Goodwill and intangible assets Related Party Transactions [Abstract] Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Premia Holdings Ltd. Investee [Member] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Percentage ownership Equity Method Investment, Ownership Percentage Purchases of other investments Payments to Acquire Other Investments Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses Reinsurance Recoverables, Including Reinsurance Premium Paid Funds held liability Funds Held under Reinsurance Agreements, Liability SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract] Reinsurance SEC Schedule, 12-17, Insurance Companies, Reinsurance [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Corporate bonds Corporate Bond Securities [Member] Collateral [Axis] Collateral [Axis] Collateral [Domain] Collateral [Domain] Cash and Cash Equivalents Cash and Cash Equivalents [Member] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Watford Holdings Ltd Watford Holdings Ltd [Member] Watford Holdings Ltd [Member] ACGL Parent Company [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Investable assets Investable Assets Investable Assets OTTI unrealized losses at current fair value Other Than Temporary Impairment Losses Investments Available For Sale Securitiess At Current Fair Value The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income, but reflecting subsequent changes in fair value subsequent to the impairment measurement date. Continuous unrealized loss, qualitative disclosures: Available-for-sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract] Number of positions in an unrealized loss position (lots) Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Total number of positions (lots) Available For Sale Securities Qualitative Disclosure Number Of Positions Disclosure regarding factors used to determine that the impairment of securities categorized as available for sale securitie.This item contains disclosure of the number of investment positions held before consideration of those determined to be temporarily impaired. Largest single loss Available For Sale Securities Qualitative Disclosure Largest Single Loss Disclosure regarding factors used to determine that the impairment of securities categorized as available for sale securitie.This item contains disclosure of the largest single loss position in a single lot as of the balance sheet date. OTTI Unrealized Losses Other than Temporary Impairment Losses, Investments, Available-for-sale Securities Equity securities, at fair value Equity Securities, FV-NI Equity Method Investment, Financial Statement, Reported Amounts [Abstract] Equity Method Investment, Financial Statement, Reported Amounts [Abstract] Equity in net income (loss) of investment funds accounted for using the equity method Equity method investments, time lag for reporting - low end of range Equity Method Investments Time Lag For Reporting Low End of Range Equity Method Investments Time Lag For Reporting Low End of Range Equity method investments, time lag for reporting - high end of range Equity Method Investments Time Lag For Reporting High End of Range Equity Method Investments Time Lag For Reporting High End of Range Short Duration Contracts Disclosure [Abstract] Short Duration Contracts Disclosure [Abstract] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Insurance Insurance Segment [Member] The insurance segment consists of the Company's insurance underwriting subsidiaries.= which primarily write on both an admitted and non-admitted basis. Reinsurance Reinsurance Segment [Member] The reinsurance segment consists of the Company's reinsurance underwriting subsidiaries. The reinsurance segment generally seeks to write significant lines on specialty cproperty and casualty reinsurance contracts. Mortgage Mortgage Segment [Member] Mortgage Segment [Member] Consolidating adjustments and eliminations Consolidation, Eliminations [Member] Other Corporate and Other [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Property, energy, marine and aviation Property Energy Marine And Aviation [Member] -- None. No documentation exists for this element. -- Third party occurrence business Third Party Occurrence Business [Member] Third Party Occurrence Business [Member] Third party claims-made business Third Party Claims-Made Business [Member] Third Party Claims-Made Business [Member] Casualty Casualty [Member] -- None. No documentation exists for this element. -- Multi-line and other specialty Multiline And Other Specialty [Member] Multiline And Other Special [Member] Property catastrophe Property Catastrophe [Member] -- None. No documentation exists for this element. -- Property excluding property catastrophe Property Excluding Property Catastrophe [Member] -- None. No documentation exists for this element. -- Marine and aviation Marine And Aviation [Member] -- None. No documentation exists for this element. -- Other specialty Other Specialty [Member] -- None. No documentation exists for this element. -- Other short duration lines not included Short-Duration Insurance, Other [Member] Total for short duration lines Short Duration Lines Of Business [Member] Short Duration Lines Of Business [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Loss portfolio transfer Loss Portfolio Transfer [Member] Loss Portfolio Transfer Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] Liabilities for losses and loss adjustment expenses, net of reinsurance Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net Unpaid losses and loss adjustment expenses recoverable Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Lines other than short duration Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item Discounting Short-duration Insurance Contract, Discounted Liability, Discount Unallocated claims adjustment expenses Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense Total reconciling items Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items Reserve for losses and loss adjustment expenses Liability for Claims and Claims Adjustment Expense Shareholders' Equity Stockholders' Equity Note Disclosure [Text Block] Premiums Written Premiums Written, Net [Abstract] Direct Direct Premiums Written Assumed Assumed Premiums Written Ceded Ceded Premiums Written Net premiums written Premiums Earned Premiums Earned, Net [Abstract] Direct Direct Premiums Earned Assumed Assumed Premiums Earned Ceded Ceded Premiums Earned Losses and Loss Adjustment Expenses Policyholder Benefits and Claims Incurred, Net [Abstract] Direct Policyholder Benefits and Claims Incurred, Direct Assumed Policyholder Benefits and Claims Incurred, Assumed Ceded Policyholder Benefits and Claims Incurred, Ceded Net Share-based Payment Arrangement [Abstract] Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Risk free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected option life Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Expected income tax expense (benefit) computed on pre-tax income at weighted average income tax rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Addition (reduction) in income tax expense (benefit) resulting from: Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount [Abstract] Tax-exempt investment income Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount Meals and entertainment Effective Income Tax Rate Reconciliation, Nondeductible Expense, Meals and Entertainment, Amount State taxes, net of U.S. federal tax benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Foreign branch taxes Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount Prior year adjustment Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount Foreign exchange gains & losses Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount Changes in applicable tax rate Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Dividend withholding taxes Income Tax Reconciliation Dividend Withholding Taxes The portion of the difference, between total income tax expense or benefit as reported in the Income Statement for the year/accounting period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations, that is attributable to withholding taxes on dividends. Change in valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Contingent consideration Reconciliation Of Effective Tax Rate, Contingent Consideration Reconciliation Of Effective Tax Rate, Contingent Consideration Share based compensation Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount Other Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Remaining useful lives of intangible assets Finite-Lived Intangible Assets, Remaining Amortization Period Comprehensive Income Note Disclosure [Abstract] Comprehensive Income Note Disclosure [Abstract] Before tax amount: Other Comprehensive Income (Loss), before Tax [Abstract] Unrealized holding gains (losses) arising during period, before tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax Less reclassification of net realized gains (losses) included in net income, before tax Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax Foreign currency translation adjustments, before tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax Other comprehensive income (loss), before tax Other Comprehensive Income (Loss), before Tax Tax expense (benefit): Other Comprehensive Income (Loss), Tax [Abstract] Unrealized holding gains (losses) arising during period, tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Less reclassification of net realized gains (losses) included in net income, tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Foreign currency translation adjustments, tax Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax Other comprehensive income (loss), tax Other Comprehensive Income (Loss), Tax Net of tax amount: Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized holding gains (losses) arising during period, net of tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Less reclassification of net realized gains (losses) included in net income, net of tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Foreign currency translation adjustments, net of tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Net current period other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Schedule Of Restricted Assets [Table] Schedule Of Restricted Assets [Table] Provides details of restricted assets, by type of restriction. Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Restricted Assets Restriction Type [Axis] Restricted Assets Restriction Type [Axis] The types of restrictions that are imposed on the Company. Restricted Assets Restriction Type [Domain] Restricted Assets Restriction Type [Domain] The types of restrictions that are imposed on the Company. Collateral or guarantees - affiliated transactions Collateral or guarantees - affiliated transactions [Member] -- None. No documentation exists for this element. -- Collateral or guarantees - third party agreements Collateral or guarantees - third party agreements [Member] -- None. No documentation exists for this element. -- Deposits with US regulatory authorities Deposits with US regulatory authorities [Member] -- None. No documentation exists for this element. -- Deposits with non-US regulatory authorities Deposits with non-US regulatory authorities [Member] -- None. No documentation exists for this element. -- Restricted Assets [Line Items] Restricted Assets [Line Items] -- None. No documentation exists for this element. -- Assets pledged as collateral Other Restricted Assets Restricted assets Restricted Assets Assets, consisting primarily of fixed maturities, that the Company is required to maintain with various regulatory authorities to support its insurance and reinsurance operations. The Company's insurance and reinsurance subsidiaries maintain assets in trust accounts as collateral for insurance and reinsurance transactions with affiliated companies and also have investments in segregated portfolios primarily to provide collateral or guarantees for letters of credit to third parties. Condensed Financial Information Disclosure [Abstract] Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Reportable legal entities Reportable Legal Entities [Member] Arch Capital (Parent Guarantor) Arch-U.S. (Subsidiary Issuer) Subsidiary Issuer [Member] Other Arch Capital Subsidiaries Subsidiaries [Member] Condensed Balance Sheet Statements, Captions [Line Items] Condensed Balance Sheet Statements, Captions [Line Items] Percentage ownership of subsidiary Subsidiary Investee Percentage Ownership Subsidiary Investee Percentage Ownership Assets Assets [Abstract] Total investments Investments Cash Cash and Cash Equivalents, at Carrying Value Investment in subsidiaries Investment In Subsidiaries Investment in subsidiaries which is eliminated upon consolidation. Due from subsidiaries and affiliates Advances to Affiliate Premiums receivable Premiums Receivable, Net Contractholder receivables Contractholder Deductibles Receivable -- None. No documentation exists for this element. -- Ceded unearned premiums Prepaid Reinsurance Premiums Deferred acquisition costs Deferred Policy Acquisition Cost Goodwill and intangible assets Intangible Assets, Liabilities, and Goodwill Net Net amount of intangible assets, intangible liabilities and goodwill. Other assets Prepaid Expense and Other Assets Total assets Assets Liabilities Liabilities [Abstract] Unearned premiums Unearned Premiums Reinsurance balances payable Reinsurance Payable Contractholder payables Contract Holder Payables Contract Holder Payables Collateral held for insured obligations Policyholder Contract Deposit Senior notes Senior Notes, Noncurrent Revolving credit agreement borrowings Long-term Line of Credit Due to subsidiaries and affiliates Due to Affiliate Other liabilities Accrued Liabilities and Other Liabilities Total liabilities Liabilities Redeemable noncontrolling interests Redeemable Noncontrolling Interest, Equity, Fair Value Shareholders’ Equity Stockholders' Equity Attributable to Parent [Abstract] Total shareholders' equity available to Arch Stockholders' Equity Attributable to Parent Non-redeemable noncontrolling interests Stockholders' Equity Attributable to Noncontrolling Interest Total shareholders' equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total liabilities, noncontrolling interests and shareholders' equity Liabilities and Equity Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other assets Other Assets [Member] T&L Bonds U.S. Mortgage Guaranty Tax and Loss Bonds [Member] U.S. Mortgage Guaranty Tax and Loss Bonds [Member] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Australia AUSTRALIA Hong Kong HONG KONG Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Tax Cuts and Jobs Act, Income Tax Expense (Benefit) Tax Cuts and Jobs Act, Income Tax Expense (Benefit) Current income taxes payable Accrued Income Taxes, Current US mortgage guaranty tax and loss bonds US Government Agencies Securities, at Carrying Value Basis of presentation Basis of Accounting, Policy [Policy Text Block] Use of estimates Use of Estimates, Policy [Policy Text Block] Premiums Insurance Premiums Revenue Recognition, Policy [Policy Text Block] Acquisition costs Deferred Policy Acquisition Costs, Policy [Policy Text Block] Deposit accounting Deposit Contracts, Policy [Policy Text Block] Retroactive accounting Retroactive Accounting Policy [Policy Text Block] Disclosure of accounting policy for those reinsurance contracts that contain both prospective and retrospective elements related to the underlying contracts. Reinsurance ceded Reinsurance Accounting Policy [Policy Text Block] Cash Cash and Cash Equivalents, Policy [Policy Text Block] Investments Investment, Policy [Policy Text Block] Derivative instruments Derivatives, Methods of Accounting, Derivatives Not Designated or Qualifying as Hedges [Policy Text Block] Reserves for losses and loss adjustment expenses Unpaid Policy Claims and Claims Adjustment Expense, Policy [Policy Text Block] Contractholders receivables and payables Contractholders Payable Receivable [Policy Text Block] The policy for accounting for those insurance contracts that feature large deductibles, primarily in construction and national accounts casualty lines. Foreign exchange Foreign Currency Transactions and Translations Policy [Policy Text Block] Income taxes Income Tax, Policy [Policy Text Block] Share-based payment arrangements Share-based Payment Arrangement [Policy Text Block] Guaranty fund and other related assessments Guaranty Fund And Other Assessments Policy [Policy Text Block] Description of the Company's accounting policy for guaranty fund liabilities and other related assessments. Treasury shares Treasury Shares Policy [Policy Text Block] Description of the policy employed by the Company in accounting for treasury share transactions. Goodwill and intangible assets Goodwill and Intangible Assets, Policy [Policy Text Block] Recent accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Senior Notes Senior Notes [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 7.350% senior notes due 2034 Senior Notes Due 2034 [Member] Senior Notes Due 2034 5.144% senior notes due 2043 Senior Notes Due 2043 [Member] Senior Notes Due 2043 [Member] 4.011% senior notes due 2026 Senior Notes Due 2026 [Member] Senior Notes Due 2026 [Member] 5.031% senior notes due 2046 Senior Notes Due 2046 [Member] Senior Notes Due 2046 [Member] Watford notes Watford Notes [Member] Watford Notes [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Stated interest rate (percentage) Debt Instrument, Interest Rate, Stated Percentage Principal amount Debt Instrument, Face Amount Carrying amount Proceeds from issuance of senior notes Proceeds from Issuance of Senior Long-term Debt Payments to Acquire Notes Receivable Payments to Acquire Notes Receivable SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract] SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Table] SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Table] Underwriting segments Operating Segments [Member] SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] Deferred Acquisition Costs SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Deferred Policy Acquisition Cost Reserves for Losses and Loss Adjustment Expenses SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Liability for Future Policy Benefit, Loss, Claim and Loss Expense Unearned Premiums SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Unearned Premium Net Premiums Earned SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Premium Revenue Net Losses and Loss Adjustment Expenses Incurred SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Benefit, Claim, Loss and Settlement Expenses Amortization of Deferred Acquisition Costs SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Cost Other Operating Expenses SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Other Operating Expense Net Premiums Written SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Premium Written Reinsurance Reinsurance [Text Block] 2019 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five 2024 and thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Finite-Lived Intangible Assets, Net Reconciliation of beginning and ending amount of unrecognized tax benefits Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Balance at beginning of year Unrecognized Tax Benefits Additions based on tax positions related to the current year Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Additions for tax positions of prior years Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Reductions for tax positions of prior years Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Settlements Unrecognized Tax Benefits, Change Resulting From Settlements With Taxing Authorities Amount of increase (decrease) in unrecognized tax benefits resulting from settlements with taxing authorities. Balance at end of year Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] Analysis of losses and loss adjustment expenses and reconciliation of beginning and ending reserve balances Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Quarterly Financial Information Disclosure [Abstract] Summary of condensed quarterly financial information Quarterly Financial Information [Table 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] Award Type [Axis] Award Type [Axis] Award Type [Domain] Award Type [Domain] Performance Common Shares and Units Performance Shares [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Performance shares and units granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Statement of Cash Flows [Abstract] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] 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 [Abstract] Net realized (gains) losses Net Realized Gains Losses This item represents the net realized gains and/or losses on investments included in earnings for the period, excluding any losses recognized for other-than-temporary impairments of the subject investments. Net impairment losses recognized in earnings Equity in net income or loss of investments accounted for using the equity method and other income or loss Equity In Net Income Loss Of Investment Funds Accounted For Using Equity Method And Other Income Loss This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. It also includes other income derived by the entity which is not classified elsewhere. Share-based compensation Share-based Payment Arrangement, Noncash Expense Changes in: Increase (Decrease) in Operating Capital [Abstract] Reserve for losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable Increase Decrease In Loss And Loss Adjustment Expenses Net Of Recoverables The net increase (decrease) in the Loss and loss adjustment expense reserve, net of recoverables. Unearned premiums, net of ceded unearned premiums Increase Decrease In Unearned Premiums, Net Of Prepaid Reinsurance Premiums The net increase (decrease) in unearned premiums, net of prepaid reinsurance premiums. Premiums receivable Increase (Decrease) in Premiums Receivable Deferred acquisition costs, net Increase (Decrease) in Deferred Policy Acquisition Costs Reinsurance balances payable Increase (Decrease) in Reinsurance Payables Other items, net Increase (Decrease) in Other Operating Assets and Liabilities, Net 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] Purchases of fixed maturity investments Payments to Acquire Debt Securities, Available-for-sale Purchases of equity securities Payments to Acquire Trading Securities Held-for-investment Purchases of other investments Proceeds from the sale of fixed maturity investments Proceeds from Sale of Debt Securities, Available-for-sale Proceeds from sales of equity securities Proceeds from Sale of Debt and Equity Securities, FV-NI, Held-for-investment Proceeds from sales, redemptions and maturities of other investments Proceeds from Sale of Other Investments Proceeds from redemptions and maturities of fixed maturity investments Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-sale Net settlements of derivative instruments Payments for (Proceeds from) Derivative Instrument, Investing Activities Net (purchases) sales of short-term investments Payments for (Proceeds from) Short-term Investments Change in cash collateral related to securities lending Increase (Decrease) in Cash Collateral for Borrowed Securities Purchases of fixed assets Payments to Acquire Productive Assets Other Payments for (Proceeds from) Other Investing Activities Net cash provided by (used for) investing activities Net Cash Provided by (Used in) Investing Activities Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of preferred shares, net Proceeds from Issuance of Preferred Stock and Preference Stock Redemption of preferred shares Payments for Repurchase of Preferred Stock and Preference Stock Purchases of common shares under share repurchase program Payments for Repurchase of Common Stock Proceeds from common shares issued, net Proceeds from (Repurchase of) Equity Proceeds from borrowings Proceeds from Other Debt Repayments of borrowings Repayments of Other Debt Change in cash collateral related to securities lending Increase (Decrease) in Cash Collateral for Loaned Securities Change in third party investment in non-redeemable noncontrolling interests Proceeds from (Payments to) Noncontrolling Interests Change in third party investment in redeemable noncontrolling interests Proceeds from (Repurchase of) Redeemable Preferred Stock Dividends paid to redeemable noncontrolling interests Payments of Ordinary Dividends, Noncontrolling Interest Other Proceeds from (Payments for) Other Financing Activities Preferred dividends paid Payments of Ordinary Dividends, Preferred Stock and Preference Stock Net cash provided by (used for) financing activities Net Cash Provided by (Used in) Financing Activities Effects of exchange rate changes on foreign currency cash and restricted cash Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Increase (decrease) in cash and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash and restricted cash, beginning of year Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash and restricted cash, end of year Supplemental cash flow information Supplemental Cash Flow Information [Abstract] Income taxes paid (received) Income Taxes Paid, Net Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities Retirement Benefits [Abstract] Defined contribution plan, expense Defined Contribution Plan, Cost Options and SARs Restricted Common Shares And Restricted Units Restricted Common Shares And Restricted Units [Member] -- None. No documentation exists for this element. -- Performance Awards Employee Share Purchase Plan Employee Share Purchase Plan [Member] -- None. No documentation exists for this element. -- Share based compensation expense Share-based Payment Arrangement, Expense Share based compensation expense, net of tax Share-based Payment Arrangement, Expense, after Tax Leases [Abstract] Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Other liabilities Other Liabilities [Member] Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Operating lease term Lessee, Operating Lease, Term of Contract Operating lease costs Operating Lease, Expense Cash payments included in the measurement of lease liabilities reported in operating cash flows Operating Lease, Payments Right-of-use assets obtained in exchange for new lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Right-of-use assets Operating Lease, Right-of-Use Asset Operating lease liability Operating Lease, Liability Weighted average discount rate Operating Lease, Weighted Average Discount Rate, Percent Weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Proceeds from stock options exercised Proceeds from Stock Options Exercised Tax benefit from exercise of stock options and SARs Share-based Payment Arrangement, Exercise of Option, Tax Benefit Weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Aggregate intrinsic value of options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Components of income taxes attributable to operations Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Schedule of income or loss before income taxes by jurisdiction Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block] Reconciliation of the differences between the provision for income taxes and the expected tax provision at the weighted average tax rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Significant components of deferred income tax assets and liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Summary of operating loss carryforwards Summary of Operating Loss Carryforwards [Table Text Block] Summary of tax credit carryforwards Summary of Tax Credit Carryforwards [Table Text Block] Reconciliation of the beginning and ending amount of unrecognized tax benefits Schedule of Unrecognized Tax Benefit Roll Forward [Table Text Block] Tabular disclosure of the change in unrecognized tax benefits. Summary of open tax years potentially subject to examination, by jurisdiction Summary Of Open Tax Years Potentially Subject To Examination By Jurisdiction [Table Text Block] Tabular disclosure of open tax years that are potentially subject to examination by local tax authorities, by major jurisdiction. 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, Acquiree [Domain] Business Acquisition, Acquiree [Domain] AIG United Guaranty Insurance (Asia) Limited AIG United Guaranty Insurance (Asia) Limited [Member] AIG United Guaranty Insurance (Asia) Limited [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Total purchase price Business Combination, Consideration Transferred Levels of disaggregation Short Duration Contracts Levels Of Disaggregation [Table Text Block] Short Duration Contracts Levels Of Disaggregation [Table Text Block] Short-duration Insurance Contracts, Claims Development [Table] Short-duration Insurance Contracts, Claims Development [Table] Claims Development [Line Items] Claims Development [Line Items] Claims development tables Short-duration Insurance Contracts, Claims Development [Table Text Block] Percentage annual payout by age Short-duration Insurance Contracts, Schedule of Historical Claims Duration [Table Text Block] Reconciliation of claims development to liability Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table Text Block] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document Type Document Type Document Annual Report Document Annual Report Amendment Flag Amendment Flag Document Transition Report Document Transition Report Document Period End Date Document Period End Date Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two Entity Address, City or Town Entity Address, City or Town Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Address, Country Entity Address, Country City Area Code City Area Code Local Phone Number Local Phone Number Entity Listings [Table] Entity Listings [Table] Common shares Common Stock [Member] Depositary Series E preferred shares Series E Preferred Stock [Member] Depositary Series F preferred shares Series F Preferred Stock [Member] Entity Listings [Line Items] Entity Listings [Line Items] Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Public Float Entity Public Float Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Documents Incorporated by Reference Documents Incorporated by Reference [Text Block] Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Other Other Investment Income [Member] -- None. No documentation exists for this element. -- Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Net investment income: Investment Income, Net [Abstract] Gross investment income Investment Income, Interest and Dividend Investment expenses Investment Income, Investment Expense Net investment income Condensed Income Statement [Table] Condensed Income Statement [Table] Condensed Income Statements, Captions [Line Items] Condensed Income Statements, Captions [Line Items] Net premiums earned Other underwriting income Other Underwriting Income Loss Other Underwriting Income Loss Total expenses Income (loss) before income taxes Income Loss Before Income Taxes And Equity In Net Income Of Subsidiaries Net income (loss) relating to the Parent Company only, prior to income taxes and equity in income of subsidiaries. Income tax (expense) benefit Income (loss) before equity in net income of subsidiaries Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries Net income (loss) relating to the Parent Company only, prior to equity in income of subsidiaries but net of income taxes. Equity in net income of subsidiaries Income (Loss) from Subsidiaries, Net of Tax Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Variable Interest Entity and Noncontrolling Interest Disclosure [Abstract] Variable Interest Entity and Noncontrolling Interest Disclosure [Abstract] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Preference equity Preferred Stock [Member] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Initial capitalization amount Proceeds from Issuance or Sale of Equity Proceeds from from common shares issued, net Proceeds from Issuance of Common Stock Proceeds on issuance of preference shares Initial investment contribution amount Payments to Acquire Businesses and Interest in Affiliates Number of shares acquired Investment Owned, Balance, Shares Number of shares called by warrants Class of Warrant or Right, Number of Securities Called by Warrants or Rights Ownership percentage Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Equity method investments Equity Method Investments [Member] Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Maximum Exposure to Loss Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Aggregate commitments Investment Commitments Investment commitments primarily related to agreements entered into by the Company to invest in funds and separately managed accounts when called upon. Income Tax Disclosure [Table] Income Tax Disclosure [Table] Income Tax Disclosure [Table] Bermuda Domestic Tax Authority [Member] Other Other Country [Member] -- None. No documentation exists for this element. -- Income Tax Disclosure [Line Items] Income Tax Disclosure [Line Items] [Line Items] for Income Tax Disclosure [Table] Income (loss) before income taxes Fair Value Disclosures [Abstract] Fair value hierarchy Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Rollforward of Level 3 investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Significant Accounting Policies [Table] Significant Accounting Policies [Table] Significant Accounting Policies [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Retained earnings Retained Earnings [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] ASU 2016-02 Accounting Standards Update 2016-02 [Member] Losses occurring Losses Occurring [Member] -- None. No documentation exists for this element. -- Risks attaching Risks Attaching [Member] -- None. No documentation exists for this element. -- Significant Accounting Policies [Line Items] Significant Accounting Policies [Line Items] [Line Items] for Significant Accounting Policies [Table] Premium revenue recognition period Premium Revenue Recognition Period -- None. No documentation exists for this element. -- Securities lending rate of collateral required (percentage) Securities Lending Rate Of Collateral Required The rate of collateral required, expressed as a percentage of the fair value of loaned securities. Vesting period, share-based awards Requisite service period, share-based awards Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Roll-forward of changes in issued and outstanding Common Shares Schedule of Common Stock Outstanding Roll Forward [Table Text Block] Schedule of share repurchases Treasury Stock [Text Block] Disclosure Legal Proceedings [Abstract] Disclosure Legal Proceedings [Abstract] Legal Proceedings Legal Matters and Contingencies [Text Block] Segment Reporting [Abstract] Segment Information Segment Reporting Disclosure [Text Block] Commitments and Contingencies Disclosure [Abstract] Concentration Risk [Table] Concentration Risk [Table] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer Concentration Risk Customer Concentration Risk [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Gross Written Premiums Gross Written Premiums [Member] -- None. No documentation exists for this element. -- Aon Corporation Aon Corporation [Member] -- None. No documentation exists for this element. -- Marsh And McLennan Companies Marsh And McLennan Companies [Member] -- None. No documentation exists for this element. -- Concentration Risk [Line Items] Concentration Risk [Line Items] Concentration risk percentage Concentration Risk, Percentage Summary of fair value and cost or amortized cost of available for sale securities Available-for-sale Securities [Table Text Block] Summary of available for sale securities in a continual unrealized loss position Schedule of Unrealized Loss on Investments [Table Text Block] Contractual maturities of the Company's fixed maturities and fixed maturities pledged under securities lending arrangements Investments Classified by Contractual Maturity Date [Table Text Block] Summary of securities lending transactions accounted for as secured borrowings, with significant investment categories Schedule of Assets and Associated Liabilities Accounted for as Secured Borrowings [Table Text Block] Summary of other investments, including available for sale and fair value option components Schedule Of Other Investments [Table Text Block] Components of other investments. Summary information for investments accounted for using the equity method Equity Method Investments [Table Text Block] Summary of assets and liabilities accounted for using the fair value option Fair Value Option, Disclosures [Table Text Block] Summary of investments in limited partnership interests where the Company has a variable interest Schedule of Variable Interest Entities [Table Text Block] Components of net investment income Investment Income [Table Text Block] Summary of net realized gains (losses) Realized Gain (Loss) on Investments [Table Text Block] Summary of OTTI recognized in earnings by asset class Other Than Temporary Impairment Credit Losses [Table Text Block] Tabular disclosure of the amount of other than temporary impairment (OTTI) related to credit losses recognized in earnings in each period presented. Rollforward of the amount related to credit losses recognized in earnings for which a portion was recognized in AOCI Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table Text Block] Summary of restricted assets Restricted Assets [Table Text Block] Tabular disclosure for assets that are restricted in their use, generally by contractual agreements or regulatory requirements. Reconciliation of cash and restricted cash Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Table] Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Table] Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Repurchase Agreements and Similar Transactions, Maturity Periods [Domain] Repurchase Agreements and Similar Transactions, Maturity Periods [Domain] Overnight and Continuous Maturity Overnight and on Demand [Member] Less than 30 Days Maturity Less than 30 Days [Member] 30-90 Days Maturity 30 to 90 Days [Member] 90 Days or More Maturity Greater than 90 Days [Member] Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items] Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items] Securities lending payable Obligation to Return Securities Received as Collateral Gross amount of recognized liabilities for securities lending in offsetting disclosure in Note 10 Secured Borrowings, Gross, Difference, Amount Amounts related to securities lending not included in offsetting disclosure in Note 10 Secured Borrowings, Gross Including Not Subject to Master Netting Arrangement Collateral received under securities lending, at fair value Securities Received as Collateral Number of Options / SARs Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding, beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Forfeited or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding, end of year (in shares) Exercisable, end of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding, beginning of year, weighted average exercise price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted, weighted average exercise price Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised, weighted average exercise price Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited or expired, weighted average exercise price Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Outstanding, end of year, weighted average exercise price Exercisable, end of year, weighted average exercise price Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Weighted average contractual term, outstanding, end of year Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Weighted average contractual term, exercisable, end of year Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Aggregate intrinsic value, outstanding, end of year Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Aggregate intrinsic value, exercisable, end of year Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value United Guaranty Residential Insurance Company [Member] United Guaranty Residential Insurance Company [Member] United Guaranty Residential Insurance Company [Member] Arch Re US Arch Re US [Member] Arch Re US [Member] Arch Re Bermuda [Member] Arch Re Bermuda [Member] Arch Re Bermuda is the Company's Bermuda reinsurance and insurance subsidiary. North Carolina NORTH CAROLINA North Carolina or Wisconsin North Carolina Or Wisconsin [Member] North Carolina Or Wisconsin [Member] Dividends, percentage permitted Statutory Accounting Practices Dividends Permitted Percentage Of Statutory Capital And Surplus The percentage of total statutory capital and surplus (as shown on its previous financial year's statutory balance sheet) which may be paid out as dividends without the requirement to file an affadavit with the regulatory body stating that the Company will continue to meet required margins. Dividend required notice period Statutory Capital And Surplus Notice Period Required For Dividends Statutory Capital And Surplus Notice Period Required For Dividends Dividends available for payment in 2020 without prior regulatory approval Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval Number Of Subsidiaries Number Of Subsidiaries Number Of Subsidiaries Dividends available for payment in 2020, total Statutory Accounting Practices, Statutory Amount Available for Dividend Payments Required contingency loss reserve as a percentage of net earned premiums Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums Withdrawal restriction period Withdrawal Restriction Period Withdrawal Restriction Period Withdrawal restriction period, with prior approval Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums With Prior Approval Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums With Prior Approval Maximum risk to capital ratio Maximum Risk To Capital Ratio Maximum Risk To Capital Ratio Restricted Common Shares Restricted Stock [Member] Restricted Unit Awards Restricted Stock Units (RSUs) [Member] Unvested Shares: Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Unvested balance, beginning of year (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Restricted shares and restricted unit awards granted Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Unvested balance, end of year (in shares) Weighted Average 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] Unvested balance, beginning of year, weighted average 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 Vested, weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Forfeited, weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Unvested balance, end of year, weighted average grant date fair value Fair Value Option, Disclosures [Table] Fair Value Option, Disclosures [Table] Other investments Other investments fair value [Member] Line item in the statement of financial position in which the fair value amounts of the other investments are included. This category includes derivative instruments, hedge funds and other types of investments not classified elsewhere. Fair Value, Option, Quantitative Disclosures [Line Items] Fair Value, Option, Quantitative Disclosures [Line Items] Investments accounted for using the fair value option Investments Accounted For Using Fair Value Option Certain fixed maturity securities, equity securities and other investments held at fair value under the fair value option afforded by accounting guidance regarding the fair value option for financial assets and liabilities. Carrying amount of assets and liabilities of consolidated VIE Variable Interest Entity Condensed Balance Sheet [Table Text Block] Tabular disclosure of the carrying amount and balance sheet caption in which the assets and liabilities of the variable interest entity are shown. Summary of VIE cash flows Variable Interest Entity Cash Flow [Table Text Block] Variable Interest Entity Cash Flow [Table Text Block] Activity in non-redeemable noncontrolling interests Minority Interest Rollforward Activity [Table Text Block] Tabular disclosure of the activity in the noncontrolling interest in consolidated subsidiaries account for the period. Activity in redeemable noncontrolling interest Redeemable Noncontrolling Interest [Table Text Block] Portion of income or loss attributable to noncontrolling interests Portion of Income Or Loss Attributable to Minority Interest [Table Text Block] Portion of Income Or Loss Attributable to Minority Interest [Table Text Block] Total assets and maximum loss exposure of VIE Variable Interest Entity Maximum Exposure To Loss [Table Text Block] Variable Interest Entity Maximum Exposure To Loss [Table Text Block] Broker concentration risk Schedules of Concentration of Risk, by Risk Factor [Table Text Block] Line of Credit Facility [Table] Line of Credit Facility [Table] CMG Mortgage Insurance Company CMG Mortgage Insurance Company [Member] CMG Mortgage Insurance Company [Member] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Investment commitments Contingent consideration liability Business Combination, Contingent Consideration, Liability Estimated purchase commitments, primarily related to software and computerized systems Purchase Commitment, Remaining Minimum Amount Committed Debt and Financing Arrangements Long-term Debt [Text Block] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Line Items] Class of Stock [Line Items] Remaining share repurchase authorization Stock Repurchase Program, Remaining Authorized Repurchase Amount Treasury stock, value of shares acquired Treasury Stock, Value, Acquired, Cost Method Treasury stock, shares acquired (shares) Treasury Stock, Shares, Acquired Treasury stock, average cost per share Treasury Stock Acquired, Average Cost Per Share Cumulative number of shares acquired since inception of share repurchase program Treasury Stock Shares Acquired Cumulative Since Inception Cumulative number of shares that have been repurchased since the inception of the share repurchase program. Aggregate purchase price of shares acquired since inception of share repurchase program Treasury Stock Value Acquired Cumulative Since Inception Aggregate cumulative purchase price of common and preferred stock that have been repurchased since the inception of the share repurchase program. Common shares held in treasury (shares) Treasury Stock, Shares Common shares held in treasury, at cost Treasury Stock, Value Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Axis] Short-duration Insurance Contracts, Accident Year [Domain] Short-duration Insurance Contracts, Accident Year [Domain] 2012 Short-duration Insurance Contracts, Accident Year 2012 [Member] 2013 Short-duration Insurance Contracts, Accident Year 2013 [Member] 2014 Short-duration Insurance Contracts, Accident Year 2014 [Member] 2015 Short-duration Insurance Contracts, Accident Year 2015 [Member] 2016 Short-duration Insurance Contracts, Accident Year 2016 [Member] 2017 Short-duration Insurance Contracts, Accident Year 2017 [Member] 2018 Short-duration Insurance Contracts, Accident Year 2018 [Member] 2019 Short-Duration Insurance Contract, Accident Year 2019 [Member] Incurred losses and allocated loss adjustment expenses, net of reinsurance Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net