Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Weighted average common shares and common share equivalents outstanding Weighted Average Number of Shares Outstanding, Diluted [Abstract] Trading Arrangement: Trading Arrangement [Axis] Purchases, issuances, sales and settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements) [Abstract] Maximum Maximum [Member] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Restricted Assets [Line Items] Restricted Assets [Line Items] -- None. No documentation exists for this element. -- Statistical Measurement [Domain] Statistical Measurement [Domain] Total assets and maximum exposure to loss associated with VIEs Variable Interest Entity Maximum Exposure To Loss [Table Text Block] Variable Interest Entity Maximum Exposure To Loss [Table Text Block] Title of 12(b) Security Title of 12(b) Security Equity method investments Equity Method Investments [Member] Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-Sale [Table] Net income (loss) available to Arch common shareholders, diluted Net Income (Loss) Available to Common Stockholders, Diluted 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. Document Fiscal Year Focus Document Fiscal Year Focus Transaction costs and other Business Combination, Acquisition-Related Cost, Expense Adjustment to Compensation, Amount Adjustment to Compensation Amount Financial Instrument [Axis] Financial Instrument [Axis] Segment Reporting [Abstract] Segment Reporting [Abstract] Arch Finance Arch Capital Finance LLC [Member] Arch Capital Finance LLC Common shares ($0.0011 par, shares issued: 602.7 and 599.8) Common Stock, Value, Issued Interest expense Interest expense Interest Expense, Operating and Nonoperating 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. Restatement Determination Date Restatement Determination Date Disclosure Investment Information [Abstract] Disclosure Investment Information [Abstract] -- None. No documentation exists for this element. -- Short-Duration Insurance Contracts, Claim Duration [Domain] Short-Duration Insurance Contracts, Claim Duration [Domain] Derivative, by Nature [Axis] Derivative Instrument [Axis] Equities Fair Value Option Equities [Member] Fair Value Option Equities Reinsurance balances payable Reinsurance Payable Unrealized holding gains (losses) arising during period, tax Other Comprehensive Income (Loss) before Reclassifications, Tax Contractholder receivables Contract with Customer, Asset, Allowance for Credit Loss [Table Text Block] U.S. government and government agencies US Treasury and Government [Member] Amendment Flag Amendment Flag Consolidated Entities [Axis] Consolidated Entities [Axis] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Other assets acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets Amount of other assets acquired at the acquisition date. Issuances Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances Reinsurance Reinsurance Segment [Member] The reinsurance segment consists of the Company’s reinsurance underwriting units which offer specialty product lines on a worldwide basis. Gross Unrealized Losses Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Investment information Investment [Text Block] Asset backed securities Asset-Backed Securities [Member] Investment Type Categorization [Domain] Investments [Domain] Other comprehensive income (loss), net of deferred income tax Net of tax amount: Other Comprehensive Income (Loss), Net of Tax [Abstract] Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Table] Liability for Unpaid Claim and Claim Adjustment Expense, Change [Table] Comprehensive Income Note Disclosure [Abstract] Comprehensive Income Note Disclosure [Abstract] Comprehensive Income Note Disclosure [Abstract] Deferred income tax assets, net Increase (Decrease) in Deferred Income Taxes Consolidated Entities [Domain] Consolidated Entities [Domain] Transactions with related parties Related Party Transactions Disclosure [Text Block] Equity Method Investee Equity Method Investee [Member] Series G Depositary Share Equivalent Series G Depositary Share Equivalent [Member] Depositary shares, each representing a 1/1000th interest in a 4.55% Series G preferred share Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Net realized gains (losses) Net Realized Gains Losses Includes realized gains and losses on debt securities accounted for as available for sale, realized and unrealized changes in the fair value of equity securities and assets and liabilities accounted for using the fair value option, realized and unrealized gains and losses on derivative instruments and changes in the allowance for credit losses on financial assets, and all other. Gross Unrealized Losses Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Purchases of equity securities Payments to Acquire Equity Securities, FV-NI Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] PEO Total Compensation Amount PEO Total Compensation Amount Common shares, par value per share Common Stock, Par or Stated Value Per Share Reclassification out of accumulated other comprehensive income Reclassification out of Accumulated Other Comprehensive Income [Member] Notional value Derivative, Notional Amount Share Repurchase Program [Domain] Share Repurchase Program [Domain] Basic (shares) Weighted average common shares outstanding — basic Weighted Average Number of Shares Outstanding, Basic Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Number of reportable segments Number of Reportable Segments Deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Investment in operating affiliates Investment In Operating Affiliate Equity Method Investments in which the Company has significant influence over the operating and financial policies and are accounted for under the equity method. Rollforward of the allowance for expected credit losses of securities classified as available for sale Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table Text Block] Investments: Investments [Abstract] Due after five years through 10 years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Income (loss) before income taxes and income (loss) from operating affiliates Income (loss) before income taxes and income (loss) from operating affiliates Income Loss From Continuing Operations Before Earnings From Affiliates Income Taxes Noncontrolling Interest Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from affiliates. Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Entity Address, City or Town Entity Address, City or Town Reconciliation of cash and restricted cash Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost, Fiscal Year Maturity [Abstract] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Purchase price Business Combination, Consideration Transferred [Abstract] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Senior Notes Due 2056 Senior Notes Due 2056 [Member] Senior Notes Due 2056 Basis of accounting Basis of Accounting, Policy [Policy Text Block] Preferred dividends paid Payments of Ordinary Dividends, Preferred Stock and Preference Stock Class of Stock [Axis] Class of Stock [Axis] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Net incurred losses and loss adjustment expenses relating to losses occurring in: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Net income (loss) available to Arch common shareholders Net income (loss) available to Arch common shareholders, basic Net income (loss) available to Arch common shareholders Net Income (Loss) Available to Common Stockholders, Basic Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Allowance for expected credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss [Roll Forward] Other operating expenses Other Expenses Concentration Risk Type [Axis] Concentration Risk Type [Axis] Other changes Adjustments to Additional Paid in Capital, Other Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Unrealized holding gains (losses) arising during period, before tax OCI, before Reclassifications, before Tax, Attributable to Parent Aggregate unfunded commitments Investment 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. Investment expenses Investment Income, Investment Expense Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Common shares held in treasury, at cost (shares: 261.8 and 240.8) Treasury Stock, Value Recent accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Analysis of underwriting income or loss by segment and reconciliation to net income available to common shareholders Segment Reporting [Table Text Block] Before tax amount: Other Comprehensive Income (Loss), before Tax [Abstract] Municipal bonds Municipal Bonds [Member] Use of estimates Use of Estimates, Policy [Policy Text Block] Related Party [Axis] Related and Nonrelated Parties [Axis] Other comprehensive income (loss), net of tax Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Rollforward of Level 3 liabilities Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Futures contracts Future [Member] Net losses and loss adjustment expense reserves of acquired business Liability for Unpaid Claims and Claims Adjustment Expense, Business Combination Senior Notes Due 2043 Senior Notes Due 2043 [Member] Senior Notes Due 2043 Ceded Credit Risk, Collateralization [Domain] Ceded Credit Risk, Collateralization [Domain] Due after one year through five years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Document Period End Date Document Period End Date Changes in: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Coface Coface [Member] Coface Reserves For Losses And Loss Adjustment Expenses Reserves For Losses And Loss Adjustment Expenses [Member] Reserves For Losses And Loss Adjustment Expenses Net cash used for investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Available for sale securities: Debt Securities, Available-for-Sale, Gain (Loss) [Abstract] Aggregate principal amount Debt Instrument, Face Amount Security Exchange Name Security Exchange Name Available for sale securities, gross losses on investment sales Debt Securities, Available-for-Sale, Realized Loss Gross premiums written Premiums Written, Gross Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Due after 10 years Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10 Series F Depositary Share Equivalent Series F Depositary Share Equivalent [Member] Depositary shares, each representing a 1/1000th interest in a 5.45% Series F preferred share Reinsurance balances payable Increase (Decrease) in Reinsurance Payables Other comprehensive income (loss), tax Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Total liabilities measured at fair value Liabilities, Fair Value Disclosure Accumulated other comprehensive income (loss), net of deferred income tax Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Total gains or (losses) (realized/unrealized) - included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Estimated Fair Value - 12 Months or More Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] All Other Carriers All Other Carriers [Member] All Other Carriers Earnings per common share Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Earnings Per Share [Abstract] Earnings Per Share [Abstract] Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Document Fiscal Period Focus Document Fiscal Period Focus Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] Total liabilities Liabilities Trading Symbol Trading Symbol Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Company Selected Measure Name Company Selected Measure Name Total gains or (losses) (realized/unrealized) - included in other comprehensive income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Reinsurance recoverable Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments [Member] Purchases of fixed maturity investments Payments to Acquire Debt Securities, Available-for-Sale Less reclassification of net realized gains included in net income, net of tax Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Less reclassification of net realized gains included in net income, before tax Reclassification from AOCI, Current Period, before Tax, Attributable to Parent Denominator: Weighted Average Number of Shares Outstanding, Diluted, Adjustment [Abstract] Fair value Fair Value Measurement and Measurement Inputs, Recurring and Nonrecurring [Text Block] Purchases of other investments Payments to Acquire Other Investments Residential mortgage loans Residential Mortgage [Member] Summary of net realized gains (losses) recorded in the consolidated statements of income Derivatives Not Designated as Hedging Instruments [Table Text Block] Reserve for losses and loss adjustment expenses Reserve for losses and loss adjustment expenses at beginning of period Reserve for losses and loss adjustment expenses at end of period Liability for Claims and Claims Adjustment Expense Greysbridge Holdings Ltd Greysbridge Holdings Ltd [Member] Greysbridge Holdings Ltd Related Party Transactions [Abstract] Related Party Transactions [Abstract] Operating Activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Gross investment income Investment Income, Interest and Dividend Ceded credit risk, secured Ceded Credit Risk, Secured [Member] Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Diluted (shares) Weighted average common shares and common share equivalents outstanding — diluted Weighted Average Number of Shares Outstanding, Diluted Allowance for expected credit losses Allowance for Credit Losses [Text Block] Entity Emerging Growth Company Entity Emerging Growth Company Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Ceded credit risk Ceded Credit Risk [Table Text Block] Net assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less) Credit Loss [Abstract] Credit Loss [Abstract] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Components of net investment income Investment Income [Table Text Block] Additions (reductions) for previously recognized expected credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Maximum Exposure to Loss Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Derivative liabilities subject to master netting agreements - fair value Derivative Liability, Subject to Master Netting Arrangement, before Offset Rollforward of Level 3 assets Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Other intangible assets Other Intangible Assets [Member] Accrued investment income Accrued Investment Income Receivable Securities without single maturity date Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Amortized Cost Other items, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Award Timing Predetermined Award Timing Predetermined [Flag] Common shares held in treasury, at cost Treasury Stock, Common [Member] Schedule of tax effects allocated to each component of other comprehensive income (loss) Comprehensive Income (Loss) [Table Text Block] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-Sale [Line Items] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Remaining Coverage, Amount Remaining Coverage, Amount Remaining Coverage, Amount Amortization of share-based compensation APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Losses and loss adjustment expenses Losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] 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. Net reserve for losses and loss adjustment expenses at beginning of period Net reserve for losses and loss adjustment expenses at end of period Liability for Unpaid Claims and Claims Adjustment Expense, Net Effective tax rate on income before income taxes (percentage) Effective Income Tax Rate Reconciliation, Percent Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Net realized gains and losses: Gain (Loss) on Investments, Excluding Other than Temporary Impairments [Abstract] Stock options Incremental Common Shares Attributable to Dilutive Effect of Call Options and Warrants All Executive Categories All Executive Categories [Member] 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. Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Operating affiliates Investee [Member] Residential mortgage backed securities Residential Mortgage-Backed Securities [Member] Estimated fair value of senior notes Long-Term Debt, Fair Value Distribution relationships Distribution Rights [Member] Loss reserves, fair value adjustment Liabilities, Fair Value Adjustment Single maturity date Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Antidilutive Securities [Axis] Antidilutive Securities [Axis] Summary of (favorable) adverse net PYD by segment and tail length Schedule of Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Underwriting Ratios Insurance Ratios [Abstract] Structured securities Collateralized Securities [Member] Share transactions Shareholders' Equity and Share-Based Payments [Text Block] Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Concentration Risk Type [Domain] Concentration Risk Type [Domain] Other operating expense ratio Other Operating Expense Net Ratio Percentage Other operating expense ratio is obtained by dividing other operating expenses, net of other underwriting income (loss), by net premiums earned. Net paid losses and loss adjustment expenses relating to losses occurring in: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] 2022-2 Ltd. (Sep-22) Bellemeade 2022-2 Ltd. [Member] Bellemeade 2022-2 Ltd. Due in one year or less Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One Senior notes Senior Notes, Noncurrent Derivative Liability, Statement of Financial Position [Extensible Enumeration] Derivative Liability, Statement of Financial Position [Extensible Enumeration] Schedule of computation of basic and diluted earnings per common share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses (net of allowance for credit losses: $18 and $17) Balance at beginning of period Balance at end of period Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments Segment information Segment Reporting [Text Block] Proceeds from redemptions and maturities of fixed maturity investments Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] Expiration Date Trading Arrangement Expiration Date Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Variable interest entities Variable Interest Entity Disclosure [Text Block] Liabilities Liabilities [Abstract] Short term investments Short Term Investments Fair Value Option [Member] Short-term investments accounted for using the fair value option. Effects of exchange rate changes on foreign currency cash and restricted cash Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Restricted cash (included in ‘other assets’) Restricted Cash and Cash Equivalent Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract] Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract] Combined ratio Combined Ratio Percentage The combined ratio is defined as the sum of the loss ratio, acquisition expense ratio and the other operating expense ratio. Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Amortized Cost Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Issuances Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances Other liabilities Other liabilities, at fair value Other Liabilities Business Combination [Line Items] Business Combination [Line Items] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Security, Excluded EPS Calculation [Table] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Other (2) Other Realized And Unrealized Gains And Losses All other realized and unrealized gains and losses. Available for sale securities, gross gains on investment sales Debt Securities, Available-for-Sale, Realized Gain Due after 10 years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 Exercise Price Award Exercise Price Investing Activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Segment Reporting [Line Items] Narrative items: Segment Reporting [Line Items] Document Quarterly Report Document Quarterly Report Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Segments [Domain] Segments [Domain] Proceeds from sales of fixed maturity investments Proceeds from Sale of Debt Securities, Available-for-Sale Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Investments accounted for using the equity method Equity Method Investments Business Combination [Table] Business Combination [Table] Equity Component [Domain] Equity Component [Domain] Deferred acquisition costs Deferred Policy Acquisition Cost PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Total gains or (losses) (realized/unrealized) - included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Expenses Benefits, Losses and Expenses [Abstract] Net investment income: Investment Income, Net [Abstract] Estimated Fair Value Debt Securities, Available-for-Sale, Fair Value, Fiscal Year Maturity [Abstract] Adoption Date Trading Arrangement Adoption Date Counterparty Name [Domain] Counterparty Name [Domain] Stated interest rate of Senior Notes (as a percent) Debt Instrument, Interest Rate, Stated Percentage Allowance for credit losses on premiums receivable Balance at beginning of period Balance at end of period Premium Receivable, Allowance for Credit Loss Total shareholders' equity available to Arch Balance at beginning of period Balance at end of period Equity, Attributable to Parent Payable for securities purchased Broker-Dealer, Payable to Other Broker-Dealer and Clearing Organization Assets Assets [Abstract] Receivable for securities sold Receivable from Broker-Dealer and Clearing Organization Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Other investments, at fair value Other Investments Common dividends paid Payments of Capital Distribution Available for sale securities, amortized cost Cost or Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Insider Trading Policies and Procedures [Line Items] Goodwill, measurement period adjustment Goodwill, Measurement Period Adjustment Other Proceeds from (Payment for) Other Financing Activity Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Total net incurred losses and loss adjustment expenses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Senior Notes Due 2026 Senior Notes Due 2026 [Member] Senior Notes Due 2026 Contractholder payables Contract Holder Payables Certain insurance policies written by the Company’s insurance operations feature large deductibles, primarily in its construction and national accounts lines of business. Under such contracts, the Company is obligated to pay the claimant for the full amount of the claim. The Company is subsequently reimbursed by the policyholder for the deductible amount. These amounts are included on a gross basis in the consolidated balance sheet in contractholder payables and contractholder receivables, respectively. In the event that the Company is unable to collect from the policyholder, the Company would be liable for such defaulted amounts. Collateral, primarily in the form of letters of credit, cash and trusts, is obtained from the policyholder to mitigate the Company’s credit risk. In the instances where the Company receives collateral in the form of cash, the Company reflects it in “Collateral held for insured obligations.” Business Combination [Domain] Business Combination [Domain] Commitments and contingencies Commitments and Contingencies Disclosure [Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Antidilutive securities excluded from computation of earnings per common share (shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Entity Central Index Key Entity Central Index Key Additional paid-in capital Additional Paid in Capital, Common Stock All Adjustments to Compensation All Adjustments to Compensation [Member] Business Combination [Axis] Business Combination [Axis] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Net cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Somers Somers Group Holdings Ltd [Member] Somers Group Holdings Ltd Legal proceedings Legal Matters and Contingencies [Text Block] Prior years Prior year development (favorable) adverse Prior Year Claims and Claims Adjustment Expense Income Statement [Abstract] Income Statement [Abstract] Shareholders' Equity Equity, Attributable to Parent [Abstract] Document Transition Report Document Transition Report Contractholder receivables (net of allowance for credit losses: $7 and $7) Balance at beginning of period Balance at end of period Contractholder Deductibles Receivable Certain insurance policies written by the Company’s insurance operations feature large deductibles, primarily in its construction and national accounts lines of business. Under such contracts, the Company is obligated to pay the claimant for the full amount of the claim. The Company is subsequently reimbursed by the policyholder for the deductible amount. These amounts are included on a gross basis in the consolidated balance sheet in contractholder payables and contractholder receivables, respectively. In the event that the Company is unable to collect from the policyholder, the Company would be liable for such defaulted amounts. Collateral, primarily in the form of letters of credit, cash and trusts, is obtained from the policyholder to mitigate the Company’s credit risk. In the instances where the Company receives collateral in the form of cash, the Company reflects it in “Collateral held for insured obligations.” Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Investment, Name [Axis] Investment, Name [Axis] Preferred share dividends Dividends, Preferred Stock, Cash Fixed maturities available for sale, at fair value (amortized cost: $33,427 and $32,329; net of allowance for credit losses: $16 and $20) Fixed maturities Fixed Maturities [Member] Entity Listings [Line Items] Entity Listings [Line Items] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Premia Premia Holdings Ltd [Member] Premia Holdings Ltd Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] Mortgage Mortgage Segment [Member] The mortgage segment includes the Company’s U.S. primary mortgage insurance, U.S. credit-risk transferand international mortgage insurance and reinsurance operations. City Area Code City Area Code Allowance for credit losses: Allowance for Credit Loss [Abstract] Real estate Real Estate Funds [Member] Accumulated other comprehensive income (loss), net of deferred income tax AOCI Attributable to Parent [Member] All Individuals All Individuals [Member] Amount of increase in share repurchase authorization Share Repurchase Program, Increase Authorized Amount Amount of increase in authorized share repurchase amount. Name Forgone Recovery, Individual Name 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] Summary of net realized gains (losses), including change in allowance for credit losses on financial assets and net impairment losses recognized in earnings Realized Gain (Loss) on Investments [Table Text Block] Entity Listings [Table] Entity Listings [Table] Proceeds from common shares issued, net Proceeds From Common Shares Issued Net Amount of cash inflow (outflow) from issuance of shares under share-based payment arrangement. Includes, but is not limited to, options exercised. Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Proceeds from sales, redemptions and maturities of other investments Proceeds from Sale of Other Investments Schedule Of Other Investments [Table] Schedule Of Other Investments [Table] -- None. No documentation exists for this element. -- Statement [Line Items] Statement [Line Items] 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. Total assets Total VIE Assets Assets Entity [Domain] Entity [Domain] Private equity Private Equity Funds [Member] Underwriting income (loss) Underwriting Income Loss Segment Net premiums earned and other underwriting income less: net losses and loss expenses, acquisition costs and general and administrative expenses directly related to underwriting activities. Business Combination [Abstract] Premium receivable, write-offs during the period Premium Receivable, Allowance for Credit Loss, Writeoff Net realized gains (losses) on sales during the period Equity Securities, FV-NI, Realized Gain (Loss) Net investment income Net investment income Investment Income, Net Not Rated Not Rated [Member] Not Rated [Member] Variable Interest Entity, Not Primary Beneficiary Variable Interest Entity, Not Primary Beneficiary [Member] Fixed maturities Fixed income Fixed Income Investments [Member] Operating segments Operating Segments [Member] Issued Shares Excluding Treasury Stock Issued Shares Excluding Treasury Stock [Member] Issued Shares Excluding Treasury Stock Gross Unrealized Gains Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax Total liabilities acquired Business Combination, Recognized Liability Assumed, Liability Net (purchases) sales of short-term investments Payments for (Proceeds from) Short-Term Investments Cash and investments, at fair value Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash And Investments Amount of cash and investments acquired at the acquisition date. Securities without single maturity date Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value Unrealized holding gains (losses) arising during period, net of tax OCI, before Reclassifications, Net of Tax, Attributable to Parent Change for provision of expected credit losses Premium Receivable, Allowance for Credit Loss, Period Increase (Decrease) 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. PEO PEO [Member] Collateral or guarantees - third party agreements Collateral or guarantees - third party agreements [Member] -- None. No documentation exists for this element. -- Measurement Basis [Axis] Measurement Basis [Axis] Ceded Credit Risk, Reinsurer [Domain] Reinsurer, Name [Domain] Reinsurer concentration risk Reinsurer Concentration Risk [Member] Goodwill Goodwill Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Number of positions in an unrealized loss position (lots) Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Total investments Investments Due in one year or less Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Retained earnings Retained Earnings [Member] Unrealized appreciation (decline) in value of available-for-sale securities, net of deferred income tax AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Unrealized holding gains (losses) arising during period OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Award Timing Disclosures [Line Items] AM Best A- or better AM Best Aminus Or Better Rating [Member] A- or better rating, as rated by A.M.Best. Foreign currency translation adjustments, net of deferred income tax Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Schedule Of Restricted Assets [Table] Schedule Of Restricted Assets [Table] Provides details of restricted assets, by type of restriction. Corporate bonds Corporate Debt Securities [Member] Diluted (per share) Earnings Per Share, Diluted Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] 2022-1 Ltd. (Jan-22) Bellemeade 2022-1 Ltd. [Member] Bellemeade 2022-1 Ltd. Net foreign exchange (gains) losses and other Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss) Entity Current Reporting Status Entity Current Reporting Status Other Other Contract [Member] Net cash used for financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Other liabilities acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Liabilities Amount of other liabilities assumed at the acquisition date. Amortization of intangible assets Amortization of intangible assets Intangible Asset, Finite-Lived, Amortization Expense Stockholders' Equity Note [Abstract] Stockholders' Equity Note [Abstract] Allocation of purchase price to acquired assets and liabilities Business Combination [Table Text Block] Adjustments to reconcile net income (loss) to net cash provided by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Ceded Credit Risk, Reinsurer [Axis] Reinsurer, Name [Axis] Property, energy, marine and aviation Property, Energy, Marine and Aviation Product Line [Member] Property, Energy, Marine and Aviation Product Line 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. Ceded Credit Risk [Line Items] Ceded Credit Risk [Line Items] Total assets acquired Business Combination, Recognized Asset Acquired, Asset Net premiums written Net premiums written Premiums Written, Net Less reclassification of net realized gains included in net income, tax Reclassification from AOCI, Current Period, Tax Financing Activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Summary of other investments, at fair value Investment [Table Text Block] Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Financial Instruments [Domain] Financial Instruments [Domain] Tax expense (benefit): Other Comprehensive Income (Loss), Tax [Abstract] Ceded unearned premiums Prepaid Reinsurance Premiums Summary of investments accounted for using equity method Equity Method Investments [Table Text Block] Corporate expenses Segment Other Cost And Expense Operating Operating cost and expense items that are associated with the entity's normal revenue producing operation, for segment presentation. Derivative assets, at fair value Asset derivatives - estimated fair value Derivative Asset Numerator: Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Other underwriting income Other Income Intangible Asset, Finite-Lived, Class [Axis] Intangible Asset, Finite-Lived, Class [Axis] Derivative offsetting Offsetting [Abstract] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] 2021-3 Ltd. (Sep-21) Bellemeade 2021-3 Ltd [Member] Bellemeade 2021-3 Ltd Underlying Securities Award Underlying Securities Amount Estimated Fair Value Estimate of Fair Value Measurement [Member] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Repayments of borrowings Early Repayment of Senior Debt Derivative Asset, Statement of Financial Position [Extensible Enumeration] Derivative Asset, Statement of Financial Position [Extensible Enumeration] Reserves for losses and loss adjustment expenses Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reserves For Losses And Loss Adjustment Expenses Amount of reserves for losses and loss adjustment expenses assumed at the acquisition date. Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract] Non-cumulative preferred shares Preferred Stock [Member] Other Payment for (Proceeds from) Other Investing Activity Schedule of Gain (Loss) on Securities [Table] Schedule of Gain (Loss) on Securities [Table] Segment Reporting [Table] Segment Reporting [Table] Schedule of Related Party Transactions, by Related Party [Table] Related Party Transaction [Table] Time lag for reporting Equity Method Investments Reporting Lag Equity Method Investments Reporting Lag Schedule of Equity Method Investments [Table] Equity Method Investment [Table] Income (loss) from operating affiliates Income Loss From Operating Affiliates Equity Method Income Loss From Operating Affiliates Equity Method Summary of fair value and cost or amortized cost of available for sale securities Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] Deposits with US regulatory authorities Deposits with US regulatory authorities [Member] -- None. No documentation exists for this element. -- Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Balance at beginning of period Balance at end of period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Derivative instruments Net realized gains (losses) on derivative instruments Derivative, Gain (Loss) on Derivative, Net Estimated Fair Value - Less than 12 Months Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Infrastructure Other Investments Infrastructure [Member] Other Investments Infrastructure [Member] Current Fiscal Year End Date Current Fiscal Year End Date Net realized (gains) losses Net Realized (Gains) Losses Cash Flow Statement Adjustment to reconcile net income to cash provided by (used in) operating activities that is related to net realized (gains) and losses. Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Derivative [Line Items] Derivative [Line Items] Acquisition Business Combination [Text Block] Non-U.S. government securities Debt Security, Government, Non-US [Member] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Entity File Number Entity File Number Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Contractual maturities of the Company's fixed maturities Investments Classified by Contractual Maturity Date [Table Text Block] Equity securities Equities Equity Securities [Member] 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] Purchases, issuances, sales and settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases, (Sales), Issuances, (Settlements) [Abstract] Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Common shares issued (shares) Common Stock, Shares, Issued Measure: Measure [Axis] Balance at beginning of period Balance at end of period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Summary of investments in limited partnership interests where the Company has a variable interest Schedule of Variable Interest Entities [Table Text Block] Common shares held in treasury (shares) Treasury Stock, Common, Shares Net premiums earned Net premiums earned Premiums Earned, Net Allowance for credit losses on investments Allowance for Expected Credit Losses Balance at beginning of period Balance at end of period Debt Securities, Available-for-Sale, Allowance for Credit Loss Gross Unrealized Losses - 12 Months or More Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Change in unearned premiums (Increase) Decrease in Unearned Premiums, Net Other comprehensive income (loss) Comprehensive Income (Loss) Note [Text Block] Equity Components [Axis] Equity Components [Axis] Other operating expenses Other Cost and Expense, Operating Aggregate principal amount repurchased Debt Instrument, Repurchase Amount Energy Other Investments Energy [Member] Other investments characterized as energy. Product and Service [Domain] Product and Service [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Income taxes paid (received) Income taxes paid Income Tax Paid, after Refund Received Entity Interactive Data Current Entity Interactive Data Current Available for sale Available-for-Sale Securities [Member] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Due after one year through five years Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five Investment grade fixed income Investment Grade Fixed Income [Member] -- None. No documentation exists for this element. -- Reclassification of net realized (gains) losses, included in net income (loss) Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Entity Small Business Entity Small Business Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Comprehensive income (loss) available to Arch Comprehensive Income (Loss), Net of Tax, Attributable to Parent Unearned premiums Unearned Premiums Allowance for Expected Credit Losses Premium Receivable, Allowance for Credit Loss [Roll Forward] Largest Balance Due From Any One Carrier Largest Balance Due From Any One Carrier [Member] Largest Balance Due From Any One Carrier Termination Date Trading Arrangement Termination Date Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage) Percentage Of Total Assets And Liabilities Measured At Fair Value Priced Using Non Binding Broker Quotes Represents the percentage of aggregate assets and liabilities reported on the balance sheet at period end measured at fair value which were priced using non-binding broker quotes. Useful life Intangible Asset, Finite-Lived, Acquired, Weighted-Average Amortization Period Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Current year Current Year Claims and Claims Adjustment Expense Document Type Document Type Transfers in and/or out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net 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] Gain on repurchase of senior notes Gain (Loss) on Extinguishment of Debt Fair Value as of Grant Date Award Grant Date Fair Value Deferred tax asset, goodwill and intangibles Deferred Tax Assets, Goodwill and Intangible Assets Stated redemption price (as a percent) Debt Instrument, Redemption Price, Percentage Summary of available for sale securities in an unrealized loss position Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Property other than property catastrophe Property Other Than Property Catastrophe [Member] Property Other Than Property Catastrophe Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Remaining authorized repurchase amount Share Repurchase Program, Remaining Authorized, Amount Unearned premiums Business Combination, Recognized Liability Assumed, Deferred Revenue, Current Foreign currency translation adjustments Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax Arrangement Duration Trading Arrangement Duration Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Nonvested restricted shares Incremental Common Shares Attributable to Dilutive Effect of Nonvested Shares with Forfeitable Dividends Local Phone Number Local Phone Number Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Other Other Restricted Assets [Member] Other Restricted Assets Gross Unrealized Losses - Less than 12 Months Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Other comprehensive income (loss), before tax Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent Allowance for credit losses on reinsurance recoverable Balance at beginning of period Balance at end of period Reinsurance Recoverable, Allowance for Credit Loss Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Commitments and contingencies (refer to Note 11) Commitments and Contingencies Total revenues Revenues Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Unrealized appreciation (decline) in value of available-for-sale investments: OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax [Abstract] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Equity in net income of investments accounted for using the equity method Equity in net (loss) income of investment funds accounted for using the equity method Income (Loss) from Equity Method Investments Gain (Loss) on Securities [Line Items] Gain (Loss) on Securities [Line Items] Authorized Share Repurchase Program Authorized Share Repurchase Program [Member] Authorized Share Repurchase Program Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] Bellemeade Re 2025-1 Ltd Bellemeade Re 2025-1 Ltd [Member] Bellemeade Re 2025-1 Ltd Executive Category: Executive Category [Axis] Schedule Of Other Investments [Line Items] Schedule Of Other Investments [Line Items] -- None. No documentation exists for this element. -- Equity in net (income) or loss of investments accounted for using the equity method and other income or loss Equity Pick Up, 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. Additions for current-period provision for expected credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss, Not Previously Recorded Single maturity date Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Fair Value Counterparty Name [Axis] Counterparty Name [Axis] Funds held asset Funds Held under Reinsurance Agreements, Asset Intangible Asset, Finite-Lived, Class [Domain] Intangible Asset, Finite-Lived, Class [Domain] Fair value hierarchy Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] All Award Types Award Type [Domain] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Schedule of Variable Interest Entities [Table] Variable Interest Entity [Table] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Credit related funds Credit Related Funds [Member] Credit Related Funds [Member] Treasury stock, shares acquired (shares) Treasury Stock, Shares, Acquired PEO Name PEO Name Travel and accident Travel And Accident Product Line [Member] Travel And Accident Product Line Total net paid losses and loss adjustment expenses Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Senior Notes Due 2036 Senior Notes Due 2036 [Member] Senior Notes Due 2036 Other investments measured at net asset value Fair Value Measured at Net Asset Value Per Share [Member] Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Line Items] Collateral or guarantees - affiliated transactions Collateral or guarantees - affiliated transactions [Member] -- None. No documentation exists for this element. -- Net income per common share and common share equivalent Earnings per common share: Net Income Per Common Share And Common Share Equivalent [Abstract] Net Income Per Common Share And Common Share Equivalent Other assets Other assets, at fair value Other Assets Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Entity Tax Identification Number Legal Entity [Axis] Legal Entity [Axis] Allowance for credit losses - investments related Provision for credit losses Debt Securities, Available-for-Sale, Allowance for Credit Loss, Period Increase (Decrease) Net settlements of derivative instruments Payments for (Proceeds from) Derivative Instrument, Investing Activities Net deferred tax asset, loss reserves and unearned premium Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Loss Reserves and Unearned Premiums Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from loss reserves and unearned premium, other than estimated credit losses. Assets Acquired Business Combination, Recognized Asset Acquired, Asset [Abstract] Liability Class [Axis] Liability Class [Axis] Shares repurchased for treasury Aggregate cost of shares repurchased Treasury Stock, Value, Acquired, Cost Method Cash and restricted cash, beginning of year Cash and restricted cash, end of period Cash and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Preferred dividends Preferred Stock Dividends, Income Statement Impact Long tailed lines Long Tailed Lines [Member] Long Tailed Lines [Member] Allowance for credit losses on contractholder receivable Balance at beginning of period Balance at end of period Contractholder Receivables Allowance For Credit Loss Amount of allowance for credit loss on contractholder receivables. Insurance Insurance Segment [Member] The insurance segment consists of the Company’s insurance underwriting units which offer specialty product lines on a worldwide basis. Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Change for provision of expected credit losses Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Details about amounts reclassified from AOCI Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Non-NEOs Non-NEOs [Member] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Disclosure Legal Proceedings [Abstract] Disclosure Legal Proceedings [Abstract] Disclosure Legal Proceedings [Abstract] Debt Instrument [Axis] Debt Instrument [Axis] Equity securities, at fair value: Equity Securities, FV-NI, Gain (Loss) [Abstract] Percentage ownership Equity Method Investment, Ownership Percentage Business acquisition, cash paid Payments to Acquire Businesses, Gross Share-based compensation Share-Based Payment Arrangement, Noncash Expense 2024-1 Ltd. (Aug-24) Bellemeade Re 2024-1 [Member] Bellemeade Re 2024-1 Stockholders' equity Stockholders' Equity, Total [Member] Fair value option Fair Value Option [Member] Investments accounted for using the fair value option. Investment, Name [Domain] Investment, Name [Domain] Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Due after five years through 10 years Debt Securities, Available-for-Sale, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Fair value and notional amount of derivatives Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Basic (per share) Earnings Per Share, Basic Purchases of common shares under share repurchase program Payments for Repurchase of Common Stock 2023-1 Ltd. (Oct-23) Bellemeade 2023-1 Ltd. [Member] Bellemeade 2023-1 Ltd. Statistical Measurement [Axis] Statistical Measurement [Axis] Insider Trading Arrangements [Line Items] Award Type Award Type [Axis] US MidCorp and Entertainment US MidCorp and Entertainment [Member] US MidCorp and Entertainment Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Lending Other Investments Lending [Member] Other investments characterized as lending. Retained earnings Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] Acquisition expenses Acquisition expenses Deferred Policy Acquisition Costs, Amortization Expense Equity securities, at fair value Equity Securities, FV-NI Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Common shares Common shares Common Stock [Member] Debt securities available for sale, at fair value Estimated Fair Value Debt Securities, Available-for-Sale Fair Value Measurement [Domain] Fair Value Measurement [Domain] Change for provision of expected credit losses Contractholder Receivables Credit Loss Expense Reversal Amount of expense (reversal of expense) for expected credit loss on contractholder receivables. Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Casualty Casualty [Member] Casualty product line Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Basis of presentation and recent accounting pronouncements Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Reclassification of prior periods Reclassification, Comparability Adjustment [Policy Text Block] Restricted Assets Restriction Type [Domain] Restricted Assets Restriction Type [Domain] The types of restrictions that are imposed on the Company. Derivative, Name [Domain] Derivative Contract [Domain] Concentration risk percentage Concentration Risk, Percentage Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Total assets and liabilities measured at fair value Total Assets And Liabilities Measured At Fair Value Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value. Allowance for Expected Credit Losses Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] Equity Awards Adjustments Equity Awards Adjustments [Member] Total liabilities and shareholders' equity Liabilities and Equity Goodwill and intangible assets Intangible Asset, Including Goodwill, after Accumulated Amortization Net deferred tax asset Deferred Tax Assets, Net Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Ceded Credit Risk [Table] Ceded Credit Risk [Table] Stock options Share-Based Payment Arrangement, Option [Member] Segments [Axis] Segments [Axis] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Proceeds from borrowings Proceeds from Issuance of Senior Long-Term Debt Related Party [Domain] Related and Nonrelated Parties [Domain] Premiums receivable Increase (Decrease) in Premiums Receivable Tabular List, Table Tabular List [Table Text Block] Unpaid losses and loss adjustment expenses recoverable Unpaid losses and loss adjustment expenses recoverable Reinsurance Recoverable for Unpaid Claims and Claims Adjustments Reserve for losses and loss adjustment expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Short-term investments available for sale, at fair value (amortized cost: $3,298 and $2,624; net of allowance for credit losses: $0 and $0) Short-term investments Short-Term Investments [Member] Other income (loss) Other income (loss) Other Operating Income (Expense), Net Name Awards Close in Time to MNPI Disclosures, Individual Name Entity Address, Country Entity Address, Country Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Premiums ceded Ceded Premiums Written Reinsurance recoverables Reinsurance Recoverable, Allowance for Credit Loss [Table Text Block] Corporate expenses General and Administrative Expense Fair Value by Liability Class [Domain] Fair Value by Liability Class [Domain] Derivative instruments Derivatives and Fair Value [Text Block] Net realized gains (losses) on derivative instruments Derivative, Gain (Loss) on Derivative, Net [Abstract] Collateral held for insured obligations Collateral Held For Insured Obligations Cash received by the Company as collateral related to contractholder payables. Non-cumulative preferred shares Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Commercial mortgage backed securities Commercial Mortgage-Backed Securities [Member] Hedging Designation [Domain] Hedging Designation [Domain] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Class of Stock [Domain] Class of Stock [Domain] Ceded Credit Risk, Collateralization [Axis] Ceded Credit Risk, Collateralization [Axis] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Interest paid Interest Paid, Excluding Capitalized Interest, Operating Activity Purchases of fixed assets Payments to Acquire Productive Assets Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales Foreign currency forward contracts Foreign Exchange Forward [Member] Hedging Designation [Axis] Hedging Designation [Axis] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Total assets measured at fair value Assets, Fair Value Disclosure Restricted Assets Restriction Type [Axis] Restricted Assets Restriction Type [Axis] The types of restrictions that are imposed on the Company. Property catastrophe Property Catastrophe [Member] -- None. No documentation exists for this element. -- Name Measure Name Allowance for credit losses - underwriting related Underwriting Related, Allowance For Credit Losses, Credit Loss Expense (Reversal) Amount of expense (reversal of expense) for expected credit loss on underwriting related assets. Income taxes Income Tax Disclosure [Text Block] Pay vs Performance Disclosure [Line Items] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Proceeds from sales of equity securities Proceeds from Sale of Equity Securities, FV-NI Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Debt Securities, Available-for-sale, Allowance for Credit Loss [Table] Debt Securities, Available-for-Sale, Allowance for Credit Loss [Table] Additional paid-in capital Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category Estimated Fair Value Debt Securities, Available-for-Sale, Unrealized Loss Position Short-Duration Insurance Contracts, Claim Duration [Axis] Short-Duration Insurance Contracts, Claim Duration [Axis] Name Trading Arrangement, Individual Name Cash Cash and Cash Equivalent Senior Notes Due 2046 Senior Notes Due 2046 [Member] Senior Notes Due 2046 Loss ratio Loss Ratio Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Company Selected Measure Amount Company Selected Measure Amount Premiums receivable Premium Receivable, Allowance for Credit Loss [Table Text Block] Not designated as hedging instrument Not Designated as Hedging Instrument [Member] Net unrealized gains (losses) on equity securities still held at reporting date Equity Securities, FV-NI, Unrealized Gain (Loss) Investments, Debt and Equity Securities [Abstract] Total assets and liabilities measured at fair value priced using non-binding broker quotes Total Assets And Liabilities Measured At Fair Value Priced Using Non Binding Broker Quotes Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value which were priced using non-binding broker quotes. Entity Address, Address Line Two Entity Address, Address Line Two Allowance for Expected Credit Losses Contract with Customer, Asset, Allowance for Credit Loss [Roll Forward] Schedule of Stock by Class [Table] Stock, Class of Stock [Table] Reductions due to disposals Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Liability derivatives - fair value Derivative liabilities - estimated fair value Derivative Liability Non-PEO NEO Non-PEO NEO [Member] Senior Notes Senior Notes [Member] Entity Address, Address Line One Entity Address, Address Line One Total expenses Benefits, Losses and Expenses Income tax (expense) benefit Income tax expense (benefit) Income Tax Expense (Benefit) Transfers in and/or out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net Net foreign exchange (gains) losses Net foreign exchange gains (losses) Gain (Loss), Foreign Currency Transaction, before Tax Effect of dilutive common share equivalents: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Award Timing Method Award Timing Method [Text Block] Continuous unrealized loss, qualitative disclosures: Available-for-Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract] Increase (decrease) in cash and restricted cash Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Premiums receivable (net of allowance for credit losses: $41 and $43) Balance at beginning of period Balance at end of period Premiums Receivable, Net Premiums receivable, net of commissions Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Premiums Receivable Amount of premiums receivable acquired at the acquisition date. Net income (loss) available to Arch Net income (loss) Net income available to Arch Net of tax Net Income (Loss) Attributable to Parent Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Statement [Table] Statement [Table] Restatement Determination Date: Restatement Determination Date [Axis] Short tailed lines Short Tailed Lines [Member] Types of general reinsurance business in which claims are generally reported and settled within a short time after the occurrence. Acquisition expense ratio Acquisition Cost Ratio Programs Programs Line Of Business [Member] Programs Line Of Business Other investments Other investments fair value option Other investments fair value option [Member] Other investments accounted for using the fair value option. Liabilities Acquired Business Combination, Recognized Liability Assumed, Liability [Abstract] Total gains or (losses) (realized/unrealized) - included in other comprehensive income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) Derivative assets subject to master netting agreements - fair value Derivative Asset, Subject to Master Netting Arrangement, before Offset Arch-U.S. Arch Capital Group (US) Inc [Member] Arch Capital Group (US) Inc Term loan investments Term Loan Investments [Member] -- None. No documentation exists for this element. -- Value of business acquired Insurance Contracts Acquired in Business Combination [Member] Revenues Revenues [Abstract] Change in fair value of assets and liabilities accounted for using the fair value option Fair Value, Option, Changes in Fair Value, Gain (Loss) 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. Other Other Investment Income [Member] -- None. No documentation exists for this element. -- Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]