Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities pledged as collateral for letters of credit and other	$ 191.0	$ 189.4
Securities and cash on deposit supporting Lloyd's business	331.2	409.2
Total restricted investments	727.5	826.1
Securities required to be maintained at Lloyd's	397.9
Argo Re
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities and cash on deposit supporting Lloyd's business	179.0
Securities Deposits for Regulatory and Other Purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities on deposit for regulatory and other purposes	$ 205.3	$ 227.5

Securities and letters of credit pledged.

No definition available.

Name:	argo_SecuritiesAndLettersOfCreditPledged
Namespace Prefix:	argo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Securities And Letters Of Credit Pledged As Part Of Syndicate, Minimum Amount

+ References

No definition available.

+ Details

Name:	argo_SecuritiesAndLettersOfCreditPledgedAsPartOfSyndicateMinimumAmount
Namespace Prefix:	argo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

Name:	us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedAsCollateralAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The fair value, as of the date of each statement of financial position presented, of securities that are restricted as collateral under letter of credit agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

+ Details

Name:	us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedUnderLetterOfCreditFacilitiesAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=argo_ArgoReMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=argo_SecuritiesDepositForRegulatoryAndOtherPurposeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: