Basis of Presentation (Policies)	3 Months Ended
Basis of Presentation (Policies)	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of Argo Group International Holdings, Ltd. (“Argo Group,” “we” or the “Company”) and its subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Argo Group is an underwriter of specialty insurance and reinsurance products in the property and casualty market. The preparation of interim financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting period. The major estimates reflected in our consolidated financial statements include, but are not limited to, reserves for losses and loss adjustment expenses; reinsurance recoverables, including the reinsurance recoverables allowance for expected credit losses; estimates of written and earned premiums; reinsurance premium receivable; fair value of investments and assessment of potential impairment, including the allowance for credit losses on fixed maturity securities; valuation of goodwill and intangibles and our deferred tax asset valuation allowance. Actual results could differ from those estimates. Certain financial information that normally is included in annual financial statements, including certain financial statement footnotes, prepared in accordance with GAAP, is not required for interim reporting purposes and has been condensed or omitted. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2019, filed with the Securities and Exchange Commission ("SEC") on February 28, 2020. The interim financial information as of, and for the three months ended, March 31, 2020 and 2019 is unaudited. However, in the opinion of management, the interim information includes all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of the results presented for the interim periods. The operating results for the interim periods are not necessarily indicative of the results to be expected for the full year. All significant intercompany amounts have been eliminated in consolidation. Certain reclassifications have been made to financial information presented for prior years to conform to the current year’s presentation.
Reclassification	Certain reclassifications have been made to financial information presented for prior years to conform to the current year’s presentation.
Recently Adopted Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2016-13, “Measurement of Credit Losses on Financial Instruments” (Topic 326), commonly referred to as current expected credit losses or "CECL." ASU 2016-13 requires organizations to estimate credit losses on certain types of financial instruments, including receivables and available-for-sale debt securities, by introducing an approach based on expected losses. The expected loss approach will require entities to incorporate considerations of historical information, current information and reasonable and supportable forecasts. The updated guidance also amends the previous other-than-temporary impairment model for available-for-sale debt securities by requiring the recognition of impairments relating to credit losses through an allowance account and limits the amount of credit loss to the difference between a security’s amortized cost basis and its fair value. In addition, the length of time a security has been in an unrealized loss position will no longer impact the determination of whether a credit loss exists. The guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within the year of adoption. The guidance requires a modified retrospective transition method. We adopted the updated guidance effective January 1, 2020 using the modified retrospective approach, which resulted in a $7.9 million net of tax reduction to retained earnings. Partially offsetting this reduction of retained earnings was a $5.7 million net of tax increase in other comprehensive income representing the reclassification of unrealized investment losses to credit losses under this accounting update. The cumulative effect adjustment decreased shareholders’ equity $2.2 million. In January 2020, the FASB issued ASU 2020-01, "Investments — Equity Securities" (Topic 321), "Investments — Equity Method and Joint Ventures" (Topic 323), and "Derivatives and Hedging" (Topic 815). ASU 2020-01 clarifies that entities that apply the measurement alternative in ASC 321 should consider observable transactions that result in entities initially applying or discontinuing the use of the equity method of accounting under ASC 323. The guidance also says that certain forward contracts and purchased options on equity securities that are not deemed to be in-substance common stock under ASC 323 or accounted for as derivatives under ASC 815 are in the scope of ASC 321. The guidance is effective for public business entities ("PBEs") for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years, with early adoption is permitted. The guidance should be applied prospectively. We are currently in the process of evaluating the impact that the adoption of the ASU will have on our financial results and disclosures, but it is not expected to have a material impact upon adoption. In March 2020, the FASB issued ASU 2020-04, "Reference Rate Reform" (Topic 848). ASU 2020-04 provides temporary optional expedients and exceptions to the US GAAP guidance on contract modifications and hedge accounting to ease the financial reporting burdens of the expected market transition from the London Interbank Offered Rate (LIBOR) and other interbank offered rates to alternative reference rates, such as the Secured Overnight Financing Rate. Entities can elect not to apply certain modification accounting requirements to contracts affected by what the guidance calls reference rate reform, if certain criteria are met. An entity that makes this election would not have to remeasure the contracts at the modification date or reassess a previous accounting determination. Entities can elect various optional expedients for hedging relationships affected by reference rate reform, if certain criteria are met. The guidance is effective upon issuance and generally can be applied through December 31, 2022. We have certain debt agreements for which the interest rate is based on LIBOR rates. We are currently investigating the impacts of the transition from LIBOR to other rates on these contracts and the resulting accounting requirements, but do not expect the change in rates to have a material impact on our financial statements. Updates to Accounting Policies The following accounting policies have been updated to reflect the adoption of ASU 2016-13, as described above. Investment Impairments of Available for Sale Fixed Maturities We regularly review our investments to identify and evaluate those that may have credit impairments. For fixed maturity securities, the evaluation for credit losses is generally based on the present value of expected cash flows of the security as compared to the amortized book value, the financial condition, near-term and long-term prospects for the issuer, including industry conditions, implications of rating agency actions, the likelihood of principal and interest recoverability and whether it is more likely than not we will be required to sell the investment prior to the anticipated recovery in value. Effective January 1, 2020 with the adoption of ASU 2016-13 Financial Instruments-Credit Losses, we recognize credit losses on fixed maturities through an allowance account. For fixed maturities that we do not intend to sell or for which it is more likely than not we will not be required to sell prior to the anticipated recovery in value, we separate the credit component of the impairment from the component related to all other market factors and report the credit loss component to net realized investment gains (losses) in the Consolidated Statement of Income. The impairment related to all other market factors is reported as a separate component of shareholder’s equity in other comprehensive income (loss). The credit loss allowance account is adjusted for any additional credit losses or subsequent recoveries and the cost basis of the fixed maturity security is not adjusted. For fixed maturity securities that we intend to sell or for which it is more likely that not that we will be required to sell before an anticipated recovery in value, the full amount of the impairment is recognized in net realized investment gains (losses) in the Consolidated Statement of Income and the cost basis of the fixed maturity security is adjusted to reflect the recognized realized loss. The new cost basis is not adjusted for any recoveries in fair value. We report accrued investment income separately from fixed maturity securities and have elected to not measure an allowance for credit losses for accrued investment income. The write-off of investment income accrued for fixed maturities that have defaulted on interest payments is recognized as a loss in net realized investment gains (losses), in the period of the default, in the Consolidated Statement of Income. Reinsurance Recoverables Amounts recoverable from reinsurers are estimated in a manner consistent with the associated claim liability. We report our reinsurance recoverables net of an allowance for estimated uncollectible reinsurance. The allowance is based upon our ongoing review of amounts outstanding, length of collection periods, changes in reinsurer credit standing, disputes, applicable coverage defenses and other relevant factors. We use the rating-based method to estimate the uncollectible reinsurance reserves due to credit losses. Under this method, reinsurance credit risk is estimated by considering the reinsurers probability of default. Reinsurance recoverables are forecasted out of the assumed billing periods and a liquidation factor is applied based on the rating of the reinsurer and adjusted as needed based on our historical experience with the reinsurers. Additionally, reinsurance receivable balances are evaluated to identify any dispute risk and when required, an additional reserve is recorded. Amounts deemed to be uncollectible, including amounts due from known insolvent reinsurers, are written off against the allowance. Changes in the allowance, as well as any subsequent collections of amounts previously written off, are reported as part of underwriting expense. We evaluate and monitor the financial condition of its reinsurers under voluntary reinsurance arrangements to minimize its exposure to significant losses from reinsurer insolvencies. Premiums and Unearned Premium Reserves Premiums are recognized as revenues pro rata over the policy period. Unearned premium reserves represent the unexpired portion of policy premiums. Accrued retrospective premiums are included in premium balances receivable. Premiums receivable balances are reported net of an allowance for expected losses, both dispute and credit related. The allowance is based upon our ongoing review of amounts outstanding, historical loss data, including delinquencies and write-offs, current and forecasted economic conditions and other relevant factors. Credit risk is partially mitigated by our ability to cancel the policy if the policyholder does not pay the premium.