Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Premiums and other revenue:
Earned premiums	$ 435.0	$ 420.5
Net investment (expense) income	35.5	33.9
Fee and other income	2.1	2.3
Net realized investment (losses) gains before income taxes	(36.1)	52.5
Total revenue	436.5	509.2
Expenses:
Losses and loss adjustment expenses	280.9	237.9
Underwriting, acquisition and insurance expenses	168.0	160.2
Other corporate expenses	3.3	0.5	$ 3.3
Interest expense	7.7	8.5
Fee and other expense	1.2	1.3
Foreign currency exchange (gains) loss	(3.0)	0.7
Total expenses	458.1	409.1
(Loss) income before income taxes	(21.6)	100.1
Income tax (benefit) provision	(2.8)	8.9
Net (loss) income before equity in earnings of subsidiaries	(18.8)	91.2
Equity in undistributed earnings of subsidiaries	0.0	0.0
Net (loss) income	(18.8)	91.2
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0
Net investment (expense) income	0.0	0.0
Fee and other income	0.0	0.0
Net realized investment (losses) gains before income taxes	0.0	0.0
Total revenue	0.0	0.0
Expenses:
Losses and loss adjustment expenses	0.0	0.0
Underwriting, acquisition and insurance expenses	0.0	0.0
Other corporate expenses	0.0	0.0
Interest expense	0.0	0.0
Fee and other expense	0.0	0.0
Foreign currency exchange (gains) loss	0.0	0.0
Total expenses	0.0	0.0
(Loss) income before income taxes	0.0	0.0
Income tax (benefit) provision	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	0.0	0.0
Equity in undistributed earnings of subsidiaries	8.9	(94.4)
Net (loss) income	8.9	(94.4)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	133.5	154.0
Net investment (expense) income	10.9	10.8
Fee and other income	1.3	1.2
Net realized investment (losses) gains before income taxes	4.6	6.7
Total revenue	150.3	172.7
Expenses:
Losses and loss adjustment expenses	90.4	79.9
Underwriting, acquisition and insurance expenses	60.4	55.9
Other corporate expenses	0.0	0.0
Interest expense	2.0	2.1
Fee and other expense	0.5	0.5
Foreign currency exchange (gains) loss	(2.0)	0.4
Total expenses	151.3	138.8
(Loss) income before income taxes	(1.0)	33.9
Income tax (benefit) provision	3.1	(1.1)
Net (loss) income before equity in earnings of subsidiaries	(4.1)	35.0
Equity in undistributed earnings of subsidiaries	0.0	0.0
Net (loss) income	(4.1)	35.0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0
Net investment (expense) income	0.0	(0.7)
Fee and other income	0.0	0.0
Net realized investment (losses) gains before income taxes	(0.1)	(0.1)
Total revenue	(0.1)	(0.8)
Expenses:
Losses and loss adjustment expenses	0.0	0.0
Underwriting, acquisition and insurance expenses	5.8	0.2
Other corporate expenses	2.6	0.5
Interest expense	1.4	1.7
Fee and other expense	0.0	0.0
Foreign currency exchange (gains) loss	0.0	0.0
Total expenses	9.8	2.4
(Loss) income before income taxes	(9.9)	(3.2)
Income tax (benefit) provision	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(9.9)	(3.2)
Equity in undistributed earnings of subsidiaries	(8.9)	94.4
Net (loss) income	(18.8)	91.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	301.5	266.5
Net investment (expense) income	24.6	23.8
Fee and other income	0.8	1.1
Net realized investment (losses) gains before income taxes	(40.6)	45.9
Total revenue	286.3	337.3
Expenses:
Losses and loss adjustment expenses	190.5	158.0
Underwriting, acquisition and insurance expenses	101.8	104.1
Other corporate expenses	0.7	0.0
Interest expense	4.3	4.7
Fee and other expense	0.7	0.8
Foreign currency exchange (gains) loss	(1.0)	0.3
Total expenses	297.0	267.9
(Loss) income before income taxes	(10.7)	69.4
Income tax (benefit) provision	(5.9)	10.0
Net (loss) income before equity in earnings of subsidiaries	(4.8)	59.4
Equity in undistributed earnings of subsidiaries	0.0	0.0
Net (loss) income	$ (4.8)	$ 59.4