Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Premiums and other revenue:
Earned premiums										$ 1,410.8	$ 1,371.9	$ 1,338.1
Net investment income										115.1	88.6	106.1
Fee and other income										24.5	22.2	20.7
Net realized investment and other gains										26.1	24.1	74.5
Total revenue		$ 405.4	$ 416.7	$ 384.3	$ 370.1	$ 373.4	$ 380.2	$ 377.2	$ 376.0	1,576.5	1,506.8	1,539.4
Expenses:
Losses and loss adjustment expenses										810.1	766.1	747.4
Underwriting, acquisition and insurance expenses										547.0	536.7	537.0
Interest expense										19.6	19.0	19.9
Fee and other expense										22.4	25.8	23.5
Foreign currency exchange gain										(4.5)	(18.3)	(7.8)
Impairment of intangible assets										0.0	0.0	3.4
Total expenses										1,394.6	1,329.3	1,323.4
Income before income taxes		47.0	62.2	39.6	33.1	44.2	36.2	34.7	62.4	181.9	177.5	216.0
Provision for income taxes										35.2	14.3	32.8
Net income before equity in earnings of subsidiaries										146.7	163.2	183.2
Equity in undistributed earnings of subsidiaries										0.0	0.0	0.0
Net income		$ 32.9	$ 55.2	$ 30.9	$ 27.7	$ 41.2	$ 35.3	$ 27.9	$ 58.8	146.7	163.2	183.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums										0.0	0.0	0.0
Net investment income										37.2	40.1	40.5
Fee and other income										0.0	0.0	0.0
Net realized investment and other gains										0.6	0.0	2.0
Total revenue										37.8	40.1	42.5
Expenses:
Losses and loss adjustment expenses										0.0	0.0	0.0
Underwriting, acquisition and insurance expenses										12.8	19.0	17.3
Interest expense										1.4	1.5	2.3
Fee and other expense										0.0	0.0	0.0
Foreign currency exchange gain										0.0	0.0	0.0
Impairment of intangible assets												0.0
Total expenses										14.2	20.5	19.6
Income before income taxes										23.6	19.6	22.9
Provision for income taxes										0.0	0.0	0.0
Net income before equity in earnings of subsidiaries										23.6	19.6	22.9
Equity in undistributed earnings of subsidiaries										123.1	143.6	160.3
Net income										146.7	163.2	183.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums										497.3	497.3	461.0
Net investment income										78.9	59.0	76.2
Fee and other income										21.4	19.0	17.6
Net realized investment and other gains										50.0	31.8	67.4
Total revenue										647.6	607.1	622.2
Expenses:
Losses and loss adjustment expenses										290.4	304.2	285.6
Underwriting, acquisition and insurance expenses										204.4	198.4	199.9
Interest expense										15.8	15.3	15.2
Fee and other expense										21.7	23.3	20.0
Foreign currency exchange gain										0.2	1.0	0.4
Impairment of intangible assets												3.4
Total expenses										532.5	542.2	524.5
Income before income taxes										115.1	64.9	97.7
Provision for income taxes										33.6	12.9	27.5
Net income before equity in earnings of subsidiaries										81.5	52.0	70.2
Equity in undistributed earnings of subsidiaries										0.0	0.0	0.0
Net income										81.5	52.0	70.2
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	[1]									913.5	874.6	877.1
Net investment income	[1]									40.0	30.5	(10.6)
Fee and other income	[1]									3.1	3.2	3.1
Net realized investment and other gains	[1]									(24.5)	(7.7)	7.1
Total revenue	[1]									932.1	900.6	876.7
Expenses:
Losses and loss adjustment expenses	[1]									519.7	461.9	461.8
Underwriting, acquisition and insurance expenses	[1]									329.8	319.3	319.8
Interest expense	[1]									2.4	2.2	2.7
Fee and other expense	[1]									0.7	2.5	3.5
Foreign currency exchange gain	[1]									(4.7)	(19.3)	(8.2)
Impairment of intangible assets	[1]											0.0
Total expenses	[1]									847.9	766.6	779.6
Income before income taxes	[1]									84.2	134.0	97.1
Provision for income taxes	[1]									1.6	1.4	5.3
Net income before equity in earnings of subsidiaries	[1]									82.6	132.6	91.8
Equity in undistributed earnings of subsidiaries	[1]									0.0	0.0	0.0
Net income	[1]									82.6	132.6	91.8
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	[2]									0.0	0.0	0.0
Net investment income	[2]									(41.0)	(41.0)	0.0
Fee and other income	[2]									0.0	0.0	0.0
Net realized investment and other gains	[2]									0.0	0.0	(2.0)
Total revenue	[2]									(41.0)	(41.0)	(2.0)
Expenses:
Losses and loss adjustment expenses	[2]									0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	[2]									0.0	0.0	0.0
Interest expense	[2]									0.0	0.0	(0.3)
Fee and other expense	[2]									0.0	0.0	0.0
Foreign currency exchange gain	[2]									0.0	0.0	0.0
Impairment of intangible assets	[2]											0.0
Total expenses	[2]									0.0	0.0	(0.3)
Income before income taxes	[2]									(41.0)	(41.0)	(1.7)
Provision for income taxes	[2]									0.0	0.0	0.0
Net income before equity in earnings of subsidiaries	[2]									(41.0)	(41.0)	(1.7)
Equity in undistributed earnings of subsidiaries	[2]									(123.1)	(143.6)	(160.3)
Net income	[2]									$ (164.1)	$ (184.6)	$ (162.0)

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

[2] Includes all Argo Group parent company eliminations.