Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Premiums and other revenue:
Earned premiums		$ 399.1	$ 344.9	$ 778.5	$ 689.8
Net investment (expense) income		43.6	35.7	74.1	56.9
Fee and other income		3.8	5.8	7.4	12.6
Net realized investment and other gains (losses)		4.5	(2.1)	19.1	(4.9)
Total revenue		451.0	384.3	879.1	754.4
Expenses:
Losses and loss adjustment expenses		230.6	196.6	453.1	388.2
Underwriting, acquisition and insurance expenses		154.7	133.0	308.3	265.6
Interest expense		7.0	4.9	12.9	9.7
Fee and other expense		3.3	5.7	7.4	12.2
Foreign currency exchange (gains) loss		4.6	4.5	3.9	6.0
Total expenses		400.2	344.7	785.6	681.7
Income before income taxes		50.8	39.6	93.5	72.7
Provision for income taxes		4.8	8.7	10.8	14.1
Net (loss) income before equity in earnings of subsidiaries		46.0	30.9	82.7	58.6
Equity in undistributed earnings of subsidiaries		0.0	0.0	0.0	0.0
Net income		46.0	30.9	82.7	58.6
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		(0.4)	(0.5)	(2.1)	(1.1)
Fee and other income		0.0	0.0	0.0	0.0
Net realized investment and other gains (losses)		0.0	0.2	0.5	0.2
Total revenue		(0.4)	(0.3)	(1.6)	(0.9)
Expenses:
Losses and loss adjustment expenses		0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses		3.8	1.1	12.2	6.3
Interest expense		1.1	0.4	1.4	0.7
Fee and other expense		0.0	0.0	0.0	0.0
Foreign currency exchange (gains) loss		0.0	0.0	0.0	0.0
Total expenses		4.9	1.5	13.6	7.0
Income before income taxes		(5.3)	(1.8)	(15.2)	(7.9)
Provision for income taxes		0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries		(5.3)	(1.8)	(15.2)	(7.9)
Equity in undistributed earnings of subsidiaries		51.3	32.7	97.9	66.5
Net income		46.0	30.9	82.7	58.6
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums		135.5	120.2	266.4	240.1
Net investment (expense) income		29.3	27.5	48.2	40.1
Fee and other income		2.9	4.2	5.7	9.4
Net realized investment and other gains (losses)		5.2	1.9	19.0	13.1
Total revenue		172.9	153.8	339.3	302.7
Expenses:
Losses and loss adjustment expenses		72.5	69.1	148.6	137.4
Underwriting, acquisition and insurance expenses		58.5	49.0	116.7	96.6
Interest expense		4.2	3.9	8.5	7.8
Fee and other expense		3.2	5.0	6.6	11.2
Foreign currency exchange (gains) loss		(0.1)	(0.1)	0.0	(0.1)
Total expenses		138.3	126.9	280.4	252.9
Income before income taxes		34.6	26.9	58.9	49.8
Provision for income taxes		8.1	8.3	14.5	13.7
Net (loss) income before equity in earnings of subsidiaries		26.5	18.6	44.4	36.1
Equity in undistributed earnings of subsidiaries		0.0	0.0	0.0	0.0
Net income		26.5	18.6	44.4	36.1
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	[1]	263.6	224.7	512.1	449.7
Net investment (expense) income	[1]	14.7	8.7	28.0	17.9
Fee and other income	[1]	0.9	1.6	1.7	3.2
Net realized investment and other gains (losses)	[1]	(0.7)	(4.2)	(0.4)	(18.2)
Total revenue	[1]	278.5	230.8	541.4	452.6
Expenses:
Losses and loss adjustment expenses	[1]	158.1	127.5	304.5	250.8
Underwriting, acquisition and insurance expenses	[1]	92.4	82.9	179.4	162.7
Interest expense	[1]	1.7	0.6	3.0	1.2
Fee and other expense	[1]	0.1	0.7	0.8	1.0
Foreign currency exchange (gains) loss	[1]	4.7	4.6	3.9	6.1
Total expenses	[1]	257.0	216.3	491.6	421.8
Income before income taxes	[1]	21.5	14.5	49.8	30.8
Provision for income taxes	[1]	(3.3)	0.4	(3.7)	0.4
Net (loss) income before equity in earnings of subsidiaries	[1]	24.8	14.1	53.5	30.4
Equity in undistributed earnings of subsidiaries	[1]	0.0	0.0	0.0	0.0
Net income	[1]	24.8	14.1	53.5	30.4
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	[2]	0.0	0.0	0.0	0.0
Net investment (expense) income	[2]	0.0	0.0	0.0	0.0
Fee and other income	[2]	0.0	0.0	0.0	0.0
Net realized investment and other gains (losses)	[2]	0.0	0.0	0.0	0.0
Total revenue	[2]	0.0	0.0	0.0	0.0
Expenses:
Losses and loss adjustment expenses	[2]	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	[2]	0.0	0.0	0.0	0.0
Interest expense	[2]	0.0	0.0	0.0	0.0
Fee and other expense	[2]	0.0	0.0	0.0	0.0
Foreign currency exchange (gains) loss	[2]	0.0	0.0	0.0	0.0
Total expenses	[2]	0.0	0.0	0.0	0.0
Income before income taxes	[2]	0.0	0.0	0.0	0.0
Provision for income taxes	[2]	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	[2]	0.0	0.0	0.0	0.0
Equity in undistributed earnings of subsidiaries	[2]	(51.3)	(32.7)	(97.9)	(66.5)
Net income	[2]	$ (51.3)	$ (32.7)	$ (97.9)	$ (66.5)

[1] Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.

[2] Includes all Argo Group parent company eliminations.