Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Premiums and other revenue:
Earned premiums	$ 451.5	$ 446.9	$ 1,303.7	$ 1,279.3
Net investment (expense) income	40.2	34.5	116.9	103.7
Fee and other income	1.9	3.3	6.3	7.2
Net realized investment (losses) gains	(6.2)	14.2	58.3	9.0
Total revenue	487.4	498.9	1,485.2	1,399.2
Expenses:
Losses and loss adjustment expenses	338.8	277.5	861.5	760.2
Underwriting, acquisition and insurance expenses	167.7	168.0	497.3	485.0
Interest expense	7.5	7.9	25.3	23.4
Fee and other expense	1.2	1.9	3.8	5.5
Foreign currency exchange gains	(1.6)	(1.7)	(6.2)	(2.3)
Total expenses	513.6	453.6	1,381.7	1,271.8
(Loss) income before income taxes	(26.2)	45.3	103.5	127.4
Income tax (benefit) provision	(1.1)	4.7	8.6	20.2
Net (loss) income before equity in earnings of subsidiaries	(25.1)	40.6	94.9	107.2
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(25.1)	40.6	94.9	107.2
Consolidating Adjustments [Member]
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 (losses) gains	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
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 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 (benefit) provision	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	20.2	(46.4)	(113.6)	(125.4)
Net (loss) income	20.2	(46.4)	(113.6)	(125.4)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	186.0	224.0	511.6	632.5
Net investment (expense) income	13.4	14.0	39.5	43.5
Fee and other income	1.2	1.1	3.8	3.5
Net realized investment (losses) gains	(5.6)	(1.7)	(1.8)	(6.1)
Total revenue	195.0	237.4	553.1	673.4
Expenses:
Losses and loss adjustment expenses	161.1	140.6	354.5	413.5
Underwriting, acquisition and insurance expenses	62.2	79.7	174.6	223.3
Interest expense	1.8	1.7	6.3	5.5
Fee and other expense	0.5	0.7	1.4	1.8
Foreign currency exchange gains	(1.7)	(1.7)	(6.5)	(2.5)
Total expenses	223.9	221.0	530.3	641.6
(Loss) income before income taxes	(28.9)	16.4	22.8	31.8
Income tax (benefit) provision	(2.3)	0.0	(5.9)	2.3
Net (loss) income before equity in earnings of subsidiaries	(26.6)	16.4	28.7	29.5
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(26.6)	16.4	28.7	29.5
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	(0.9)	(0.6)	(2.2)	(2.0)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment (losses) gains	(0.3)	(0.3)	(0.5)	(0.2)
Total revenue	(1.2)	(0.9)	(2.7)	(2.2)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	2.0	3.3	10.9	11.5
Interest expense	1.7	1.6	5.1	4.5
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange gains	0.0	0.0	0.0	0.0
Total expenses	3.7	4.9	16.0	16.0
(Loss) income before income taxes	(4.9)	(5.8)	(18.7)	(18.2)
Income tax (benefit) provision	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(4.9)	(5.8)	(18.7)	(18.2)
Equity in undistributed earnings of subsidiaries	(20.2)	46.4	113.6	125.4
Net (loss) income	(25.1)	40.6	94.9	107.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	265.5	222.9	792.1	646.8
Net investment (expense) income	27.7	21.1	79.6	62.2
Fee and other income	0.7	2.2	2.5	3.7
Net realized investment (losses) gains	(0.3)	16.2	60.6	15.3
Total revenue	293.6	262.4	934.8	728.0
Expenses:
Losses and loss adjustment expenses	177.7	136.9	507.0	346.7
Underwriting, acquisition and insurance expenses	103.5	85.0	311.8	250.2
Interest expense	4.0	4.6	13.9	13.4
Fee and other expense	0.7	1.2	2.4	3.7
Foreign currency exchange gains	0.1	0.0	0.3	0.2
Total expenses	286.0	227.7	835.4	614.2
(Loss) income before income taxes	7.6	34.7	99.4	113.8
Income tax (benefit) provision	1.2	4.7	14.5	17.9
Net (loss) income before equity in earnings of subsidiaries	6.4	30.0	84.9	95.9
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	$ 6.4	$ 30.0	$ 84.9	$ 95.9