Investments - Additional Information (Detail) $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Fair value of investments, assets managed on behalf of the trade capital providers \| $	$ 113.2	$ 130.8
Number of securities in an unrealized loss position, total	5,260
Number of securities in an unrealized loss position for less than one year	3,349
Number of securities in an unrealized loss position for a period of one year or greater	106

Amortized cost and fair value debt securities.

No definition available.

Name:	argo_AmortizedCostAndFairValueDebtSecuritiesAbstract
Namespace Prefix:	argo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value of Investments, Assets Managed on Behalf of the Trade Capital Provider

+ References

No definition available.

+ Details

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

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant