Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Premiums and other revenue:
Earned premiums	$ 336.1	$ 327.5	$ 661.8	$ 631.7
Net investment income	20.60	25.30	43.90	53.20
Fee income, net	0	0.2	0	0.2
Net realized investment gains (losses) and other	18.5	11.0	29.6	20.5
Total revenue	375.2	364.0	735.3	705.6
Expenses:
Losses and loss adjustment expenses	185.1	192.7	367.6	363.2
Other reinsurance-related expenses	0	4.7	0	9.8
Underwriting, acquisition and insurance expenses	136.8	124.6	265.5	251.3
Interest expense	5.1	5.1	10.1	10.0
Fee expense, net	0.1	0	1.5	0
Foreign currency exchange (gain) loss	3.4	(5.9)	3.2	(9.0)
Total expenses	330.5	321.2	647.9	625.3
(Loss) income before income taxes	44.7	42.8	87.4	80.3
Provision for income taxes	6.1	11.1	8.6	15.9
Net (loss) income before equity in earnings of subsidiaries	38.6	31.7	0	64.4
Equity in undistributed earnings of subsidiaries	0	0	78.8	0
Net income	38.6	31.7	78.8	64.4
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0	0	0	0
Net investment income	0	(0.30)	0	(0.60)
Fee income, net		0		0
Net realized investment gains (losses) and other	0	0	0	0
Total revenue	0	(0.3)	0	(0.6)
Expenses:
Losses and loss adjustment expenses	0	0	0	0
Other reinsurance-related expenses		0		0
Underwriting, acquisition and insurance expenses	0	0	(0.2)	0
Interest expense	(0.1)	(0.3)	0	(0.6)
Fee expense, net	0		0
Foreign currency exchange (gain) loss	0	0	(0.2)	0
Total expenses	(0.1)	(0.3)	0.2	(0.6)
(Loss) income before income taxes	0.1	0	0	0
Provision for income taxes	0	0	0.2	0
Net (loss) income before equity in earnings of subsidiaries	0.1	0	(90.3)	0
Equity in undistributed earnings of subsidiaries	(43.4)	(38.9)	(90.1)	(78.5)
Net income	(43.3)	(38.9)	0	(78.5)
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0	0	0	0
Net investment income	(0.10)	0	(0.20)	0
Fee income, net		0		0
Net realized investment gains (losses) and other	0	0	0	0
Total revenue	(0.1)	0	(0.2)	0
Expenses:
Losses and loss adjustment expenses	0	0	0	0
Other reinsurance-related expenses		0		0
Underwriting, acquisition and insurance expenses	4.1	6.3	1.2	12.5
Interest expense	0.6	0.8	0	1.6
Fee expense, net	0		0
Foreign currency exchange (gain) loss	0	0.1	11.3	0
Total expenses	4.7	7.2	(11.5)	14.1
(Loss) income before income taxes	(4.8)	(7.2)	0	(14.1)
Provision for income taxes	0	0	(11.5)	0
Net (loss) income before equity in earnings of subsidiaries	(4.8)	(7.2)	90.3	(14.1)
Equity in undistributed earnings of subsidiaries	43.4	38.9	78.8	78.5
Net income	38.6	31.7	0	64.4
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	117.5	108.3	229.5	216.0
Net investment income	13.80	19.60	28.80	40.50
Fee income, net		(1.0)		(1.3)
Net realized investment gains (losses) and other	13.9	18.0	25.4	25.6
Total revenue	145.2	144.9	283.7	280.8
Expenses:
Losses and loss adjustment expenses	68.8	64.7	141.3	129.0
Other reinsurance-related expenses		1.4		2.9
Underwriting, acquisition and insurance expenses	50.1	43.5	7.5	92.2
Interest expense	3.7	3.9	3.0	7.6
Fee expense, net	1.7		(0.2)
Foreign currency exchange (gain) loss	(0.1)	0	248.0	0.2
Total expenses	124.2	113.5	35.7	231.9
(Loss) income before income taxes	21.0	31.4	8.1	48.9
Provision for income taxes	5.1	8.4	27.6	11.6
Net (loss) income before equity in earnings of subsidiaries	15.9	23.0	0	37.3
Equity in undistributed earnings of subsidiaries	0	0	27.6	0
Net income	15.9	23.0	0	37.3
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	218.6	219.2	432.3	415.7
Net investment income	6.90	6.00	15.30	13.30
Fee income, net		1.2		1.5
Net realized investment gains (losses) and other	4.6	(7.0)	4.2	(5.1)
Total revenue	230.1	219.4	451.8	425.4
Expenses:
Losses and loss adjustment expenses	116.3	128.0	226.3	234.2
Other reinsurance-related expenses		3.3		6.9
Underwriting, acquisition and insurance expenses	82.6	74.8	1.6	146.6
Interest expense	0.9	0.7	(1.5)	1.4
Fee expense, net	(1.6)		3.4
Foreign currency exchange (gain) loss	3.5	(6.0)	388.8	(9.2)
Total expenses	201.7	200.8	63.0	379.9
(Loss) income before income taxes	28.4	18.6	0.5	45.5
Provision for income taxes	1.0	2.7	62.5	4.3
Net (loss) income before equity in earnings of subsidiaries	27.4	15.9	0	41.2
Equity in undistributed earnings of subsidiaries	0	0	62.5	0
Net income	$ 27.4	$ 15.9	$ 0	$ 41.2