Information Provided In Connection With Outstanding Debt Of Subsidiaries (Condensed Consolidating Statement Of (Loss) Income) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Premiums and other revenue:
Earned premiums	$ 304.3		$ 270.9		$ 871.8		$ 804.0
Net investment income	28.9		30.0		90.3		96.3
Fee income, net	3.0		1.4		4.8		1.8
Net realized investment (losses) gains	8.3		3.9		18.7		37.7
Total revenue	344.5		306.2		985.6		939.8
Expenses:
Losses and loss adjustment expenses	186.3		199.6		527.9		664.4
Other reinsurance-related expense	7.0		2.7		20.8		3.1
Underwriting, acquisition and insurance expenses	118.0		104.2		346.3		314.7
Interest expense	5.8		5.6		17.0		16.5
Foreign currency exchange (gain) loss	9.7		(1.9)		2.8		11.1
Total expenses	326.8		310.2		914.8		1,009.8
(Loss) income before income taxes	17.7		(4.0)		70.8		(70.0)
Provision for income taxes	4.3		6.8		13.8		13.3
Net (loss) income before equity in earnings of subsidiaries	13.4		(10.8)		57.0		(83.3)
Equity in undistributed earnings of subsidiaries	0		0		0		0
Net (loss) income	13.4		(10.8)		57.0		(83.3)
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0		0		0		0
Net investment income	0		0		0.1		0.1
Fee income, net	0		0		0		0
Net realized investment (losses) gains	(0.5)		0		(0.5)		0
Total revenue	(0.5)		0		(0.4)		0.1
Expenses:
Losses and loss adjustment expenses	0		0		0		0
Other reinsurance-related expense	0		0		0		0
Underwriting, acquisition and insurance expenses	10.1		4.7		21.4		16.0
Interest expense	0.1		1.3		2.9		4.1
Foreign currency exchange (gain) loss	0		0		0		0
Total expenses	10.2		6.0		24.3		20.1
(Loss) income before income taxes	(10.7)		(6.0)		(24.7)		(20.0)
Provision for income taxes	0		0		0		0
Net (loss) income before equity in earnings of subsidiaries	(10.7)		(6.0)		(24.7)		(20.0)
Equity in undistributed earnings of subsidiaries	24.1		(4.8)		81.7		(63.3)
Net (loss) income	13.4		(10.8)		57.0		(83.3)
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	108.1		111.9		327.0		330.0
Net investment income	21.2		21.6		65.3		67.3
Fee income, net	2.4		0.9		2.4		0.4
Net realized investment (losses) gains	4.2		2.2		11.8		25.7
Total revenue	135.9		136.6		406.5		423.4
Expenses:
Losses and loss adjustment expenses	78.0		77.7		220.9		223.5
Other reinsurance-related expense	0		0		0		0
Underwriting, acquisition and insurance expenses	41.8		38.0		127.9		123.6
Interest expense	4.1		3.8		11.8		11.6
Foreign currency exchange (gain) loss	(0.1)		0		0.1		(0.5)
Total expenses	123.8		119.5		360.7		358.2
(Loss) income before income taxes	12.1		17.1		45.8		65.2
Provision for income taxes	2.5		6.3		10.4		18.0
Net (loss) income before equity in earnings of subsidiaries	9.6		10.8		35.4		47.2
Equity in undistributed earnings of subsidiaries	0		0		0		0
Net (loss) income	9.6		10.8		35.4		47.2
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	196.2	[1]	159.0	[1]	544.8	[1]	474.0	[1]
Net investment income	7.8	[1]	8.8	[1]	25.1	[1]	30.2	[1]
Fee income, net	0.6	[1]	0.5	[1]	2.4	[1]	1.4	[1]
Net realized investment (losses) gains	4.6	[1]	1.7	[1]	7.4	[1]	12.0	[1]
Total revenue	209.2	[1]	170.0	[1]	579.7	[1]	517.6	[1]
Expenses:
Losses and loss adjustment expenses	108.3	[1]	121.9	[1]	307.0	[1]	440.9	[1]
Other reinsurance-related expense	7.0	[1]	2.7	[1]	20.8	[1]	3.1	[1]
Underwriting, acquisition and insurance expenses	66.1	[1]	61.5	[1]	197.0	[1]	175.1	[1]
Interest expense	1.7	[1]	0.9	[1]	2.5	[1]	2.1	[1]
Foreign currency exchange (gain) loss	9.8	[1]	(1.9)	[1]	2.7	[1]	11.6	[1]
Total expenses	192.9	[1]	185.1	[1]	530.0	[1]	632.8	[1]
(Loss) income before income taxes	16.3	[1]	(15.1)	[1]	49.7	[1]	(115.2)	[1]
Provision for income taxes	1.8	[1]	0.5	[1]	3.4	[1]	(4.7)	[1]
Net (loss) income before equity in earnings of subsidiaries	14.5	[1]	(15.6)	[1]	46.3	[1]	(110.5)	[1]
Equity in undistributed earnings of subsidiaries	0	[1]	0	[1]	0	[1]	0	[1]
Net (loss) income	14.5	[1]	(15.6)	[1]	46.3	[1]	(110.5)	[1]
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0	[2]	0	[2]	0	[2]	0	[2]
Net investment income	(0.1)	[2]	(0.4)	[2]	(0.2)	[2]	(1.3)	[2]
Fee income, net	0	[2]	0	[2]	0	[2]	0	[2]
Net realized investment (losses) gains	0	[2]	0	[2]	0	[2]	0	[2]
Total revenue	(0.1)	[2]	(0.4)	[2]	(0.2)	[2]	(1.3)	[2]
Expenses:
Losses and loss adjustment expenses	0	[2]	0	[2]	0	[2]	0	[2]
Other reinsurance-related expense	0	[2]	0	[2]	0	[2]	0	[2]
Underwriting, acquisition and insurance expenses	0	[2]	0	[2]	0	[2]	0	[2]
Interest expense	(0.1)	[2]	(0.4)	[2]	(0.2)	[2]	(1.3)	[2]
Foreign currency exchange (gain) loss	0	[2]	0	[2]	0	[2]	0	[2]
Total expenses	(0.1)	[2]	(0.4)	[2]	(0.2)	[2]	(1.3)	[2]
(Loss) income before income taxes	0	[2]	0	[2]	0	[2]	0	[2]
Provision for income taxes	0	[2]	0	[2]	0	[2]	0	[2]
Net (loss) income before equity in earnings of subsidiaries	0	[2]	0	[2]	0	[2]	0	[2]
Equity in undistributed earnings of subsidiaries	(24.1)	[2]	4.8	[2]	(81.7)	[2]	63.3	[2]
Net (loss) income	$ (24.1)	[2]	$ 4.8	[2]	$ (81.7)	[2]	$ 63.3	[2]

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

[2] Includes all Argo Group parent company eliminations