Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Investments [Line Items]
Gross realized investment gains	$ 8.9	$ 6.8	$ 40.1	$ 14.5
Gross realized investment losses	(12.2)	(6.7)	(42.3)	(13.7)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(3.3)	0.1	(2.2)	0.8
Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	0.5	0.6	5.9	0.8
Gross realized investment losses	(3.0)	(2.5)	(4.7)	(4.6)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	0.2	0.3	1.0	1.4
Gross realized investment losses	(0.6)	(0.3)	(1.0)	(0.5)
Total return strategy
Schedule of Investments [Line Items]
Gross realized investment gains	8.2	5.9	33.2	12.3
Gross realized investment losses	$ (8.6)	$ (3.9)	$ (36.6)	$ (8.6)

Foreign currency transaction loss, realized.

No definition available.

Name:	argo_ForeignCurrencyTransactionLossRealized
Namespace Prefix:	argo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894

+ Details

Name:	us-gaap_ForeignCurrencyTransactionGainLossRealized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959

+ Details

Name:	us-gaap_GainLossOnDerivativeInstrumentsNetPretax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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