Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Statement of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Premiums and other revenue:
Earned premiums	$ 334.6		$ 325.7
Net investment (expense) income	20.8		23.3
Net realized investment and other gains (loss)	16.0		11.1
Total revenue	371.4		360.1
Expenses:
Losses and loss adjustment expenses	183.7		182.5
Underwriting, acquisition and insurance expenses	129.6		128.7
Interest expense	4.9		5.0
Fee expense, net	0.4		1.4
Foreign currency exchange (gain) loss	(9.6)		(0.2)
Total expenses	309.0		317.4
Income before income taxes	62.4		42.7
Provision for income taxes	3.6		2.5
Net income (loss) before equity in earnings of subsidiaries	58.8		40.2
Equity in undistributed earnings of subsidiaries	0		0
Net income	58.8		40.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0		0
Net investment (expense) income	(0.2)		(0.1)
Net realized investment and other gains (loss)	0		0
Total revenue	(0.2)		(0.1)
Expenses:
Losses and loss adjustment expenses	0		0
Underwriting, acquisition and insurance expenses	4.7		6.0
Interest expense	0.5		0.6
Fee expense, net	0		0
Foreign currency exchange (gain) loss	0		0
Total expenses	5.2		6.6
Income before income taxes	(5.4)		(6.7)
Provision for income taxes	0		0
Net income (loss) before equity in earnings of subsidiaries	(5.4)		(6.7)
Equity in undistributed earnings of subsidiaries	64.2		46.9
Net income	58.8		40.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	135.1		112.0
Net investment (expense) income	14.5		15.0
Net realized investment and other gains (loss)	15.9		11.5
Total revenue	165.5		138.5
Expenses:
Losses and loss adjustment expenses	80.2		72.5
Underwriting, acquisition and insurance expenses	51.9		46.3
Interest expense	3.8		3.8
Fee expense, net	0.8		1.3
Foreign currency exchange (gain) loss	0.8		(0.1)
Total expenses	137.5		123.8
Income before income taxes	28.0		14.7
Provision for income taxes	5.8		3.0
Net income (loss) before equity in earnings of subsidiaries	22.2		11.7
Equity in undistributed earnings of subsidiaries	0		0
Net income	22.2		11.7
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	199.5	[1]	213.7	[1]
Net investment (expense) income	6.5	[1]	8.4	[1]
Net realized investment and other gains (loss)	0.1	[1]	(0.4)	[1]
Total revenue	206.1	[1]	221.7	[1]
Expenses:
Losses and loss adjustment expenses	103.5	[1]	110.0	[1]
Underwriting, acquisition and insurance expenses	73.0	[1]	76.4	[1]
Interest expense	0.6	[1]	0.7	[1]
Fee expense, net	(0.4)	[1]	0.1	[1]
Foreign currency exchange (gain) loss	(10.4)	[1]	(0.1)	[1]
Total expenses	166.3	[1]	187.1	[1]
Income before income taxes	39.8	[1]	34.6	[1]
Provision for income taxes	(2.2)	[1]	(0.5)	[1]
Net income (loss) before equity in earnings of subsidiaries	42.0	[1]	35.1	[1]
Equity in undistributed earnings of subsidiaries	0	[1]	0	[1]
Net income	42.0	[1]	35.1	[1]
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0	[2]	0	[2]
Net investment (expense) income	0	[2]	0	[2]
Net realized investment and other gains (loss)	0	[2]	0	[2]
Total revenue	0	[2]	0	[2]
Expenses:
Losses and loss adjustment expenses	0	[2]	0	[2]
Underwriting, acquisition and insurance expenses	0	[2]	0	[2]
Interest expense	0	[2]	(0.1)	[2]
Fee expense, net	0	[2]	0	[2]
Foreign currency exchange (gain) loss	0	[2]	0	[2]
Total expenses	0	[2]	(0.1)	[2]
Income before income taxes	0	[2]	0.1	[2]
Provision for income taxes	0	[2]	0	[2]
Net income (loss) before equity in earnings of subsidiaries	0	[2]	0.1	[2]
Equity in undistributed earnings of subsidiaries	(64.2)	[2]	(46.9)	[2]
Net income	$ (64.2)	[2]	$ (46.8)	[2]

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

[2] Includes all Argo Group parent company eliminations.