Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Premiums and other revenue:
Earned premiums	$ 445.5	$ 451.5	$ 1,313.9	$ 1,303.7
Net investment (expense) income	42.0	40.2	79.0	116.9
Fee and other income	1.7	1.9	5.8	6.3
Net realized investment and other (losses) gains before income taxes	(5.7)	(6.2)	(21.7)	58.3
Total revenue	483.5	487.4	1,377.0	1,485.2
Expenses:
Losses and loss adjustment expenses	328.9	338.8	883.0	861.5
Underwriting, acquisition and insurance expenses	164.3	164.0	493.7	485.6
Other corporate expenses	0.4	3.7	6.2	11.7
Interest expense	6.8	7.5	21.3	25.3
Fee and other expense	0.9	1.2	3.1	3.8
Foreign currency exchange losses (gains)	11.6	(1.6)	15.0	(6.2)
Total expenses	512.9	513.6	1,422.3	1,381.7
(Loss) income before income taxes	(29.4)	(26.2)	(45.3)	103.5
Income tax provision (benefit)	0.2	(1.1)	9.5	8.6
Net (loss) income before equity in earnings of subsidiaries	(29.6)	(25.1)	(54.8)	94.9
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(29.6)	(25.1)	(54.8)	94.9
Dividends on preferred shares	2.0	0.0	2.0	0.0
Net (loss) income attributable to common shareholders	(31.6)	(25.1)	(56.8)	94.9
Consolidating Adjustments
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	0.0	0.0	0.0	0.0
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment and other (losses) gains before income taxes	0.0	0.0	0.0	0.0
Total revenue	0.0	0.0	0.0	0.0
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	0.0	0.0	0.0	0.0
Other corporate expenses	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange losses (gains)	0.0	0.0	0.0	0.0
Total expenses	0.0	0.0	0.0	0.0
(Loss) income before income taxes	0.0	0.0	0.0	0.0
Income tax provision (benefit)	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Equity in undistributed earnings of subsidiaries	24.1	20.2	33.9	(113.6)
Net (loss) income	24.1	20.2	33.9	(113.6)
Dividends on preferred shares	0.0		0.0
Net (loss) income attributable to common shareholders	24.1		33.9
Other Subsidiaries and Eliminations
Premiums and other revenue:
Earned premiums	147.3	186.0	413.5	511.6
Net investment (expense) income	9.2	13.4	15.8	39.5
Fee and other income	1.1	1.2	3.8	3.8
Net realized investment and other (losses) gains before income taxes	(1.2)	(5.6)	(11.0)	(1.8)
Total revenue	156.4	195.0	422.1	553.1
Expenses:
Losses and loss adjustment expenses	117.4	161.1	313.3	354.5
Underwriting, acquisition and insurance expenses	57.1	62.2	174.0	174.6
Other corporate expenses	0.0	0.0	0.0	0.0
Interest expense	1.7	1.8	5.4	6.3
Fee and other expense	0.4	0.5	1.3	1.4
Foreign currency exchange losses (gains)	11.0	(1.7)	15.4	(6.5)
Total expenses	187.6	223.9	509.4	530.3
(Loss) income before income taxes	(31.2)	(28.9)	(87.3)	22.8
Income tax provision (benefit)	(5.8)	(2.3)	(6.4)	(5.9)
Net (loss) income before equity in earnings of subsidiaries	(25.4)	(26.6)	(80.9)	28.7
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(25.4)	(26.6)	(80.9)	28.7
Dividends on preferred shares	0.0		0.0
Net (loss) income attributable to common shareholders	(25.4)		(80.9)
Argo Group International Holdings, Ltd. (Parent Guarantor)
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	0.0	(0.9)	0.0	(2.2)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment and other (losses) gains before income taxes	0.0	(0.3)	1.3	(0.5)
Total revenue	0.0	(1.2)	1.3	(2.7)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	4.5	(1.7)	13.5	(0.7)
Other corporate expenses	0.2	3.7	5.4	11.6
Interest expense	0.8	1.7	3.3	5.1
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange losses (gains)	0.0	0.0	0.0	0.0
Total expenses	5.5	3.7	22.2	16.0
(Loss) income before income taxes	(5.5)	(4.9)	(20.9)	(18.7)
Income tax provision (benefit)	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(5.5)	(4.9)	(20.9)	(18.7)
Equity in undistributed earnings of subsidiaries	(24.1)	(20.2)	(33.9)	113.6
Net (loss) income	(29.6)	(25.1)	(54.8)	94.9
Dividends on preferred shares	2.0		2.0
Net (loss) income attributable to common shareholders	(31.6)		(56.8)
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Premiums and other revenue:
Earned premiums	298.2	265.5	900.4	792.1
Net investment (expense) income	32.8	27.7	63.2	79.6
Fee and other income	0.6	0.7	2.0	2.5
Net realized investment and other (losses) gains before income taxes	(4.5)	(0.3)	(12.0)	60.6
Total revenue	327.1	293.6	953.6	934.8
Expenses:
Losses and loss adjustment expenses	211.5	177.7	569.7	507.0
Underwriting, acquisition and insurance expenses	102.7	103.5	306.2	311.7
Other corporate expenses	0.2	0.0	0.8	0.1
Interest expense	4.3	4.0	12.6	13.9
Fee and other expense	0.5	0.7	1.8	2.4
Foreign currency exchange losses (gains)	0.6	0.1	(0.4)	0.3
Total expenses	319.8	286.0	890.7	835.4
(Loss) income before income taxes	7.3	7.6	62.9	99.4
Income tax provision (benefit)	6.0	1.2	15.9	14.5
Net (loss) income before equity in earnings of subsidiaries	1.3	6.4	47.0	84.9
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	1.3	$ 6.4	47.0	$ 84.9
Dividends on preferred shares	0.0		0.0
Net (loss) income attributable to common shareholders	$ 1.3		$ 47.0