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								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Premiums and other revenue:
Earned premiums									$ 1,186.5		$ 1,082.0		$ 1,211.6
Net investment income									118.8		125.8	[1]	133.6	[2]
Fee income, net									5.3		1.4		2.5
Net realized investment and other (losses) gains									25.7		49.2		36.8
Total revenue	350.7	344.5	318.0	323.1	318.6	306.2	336.4	297.2	1,336.3		1,258.4		1,384.5
Expenses:
Losses and loss adjustment expenses									747.6		863.1		777.5
Other reinsurance-related expenses									27.3		5.9		0
Underwriting, acquisition and insurance expenses									464.5		425.7		466.0
Interest expense									23.7		22.1		22.9
Debt extinguishment costs									2.2		0		0
Foreign currency exchange loss									4.3		3.5		(3.8)
Total expenses									1,269.6		1,320.3		1,262.6
(Loss) income before income taxes	(4.10)	17.70	25.00	28.10	8.10	(4.00)	30.70	(96.70)	66.70		(61.90)		121.90
Provision (benefit) for income taxes									14.4		20.0		35.2
Net (loss) income before equity in earnings of subsidiaries									52.3		(81.9)		86.7
Equity in undistributed earnings of subsidiaries									0		0		0
Net (loss) income	(4.7)	13.4	24.0	19.6	1.4	(10.8)	21.6	(94.1)	52.3		(81.9)		86.7
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums									0		0		0
Net investment income									0.1		100.1	[1]	17.7	[2]
Fee income, net									0		0		0
Net realized investment and other (losses) gains									(0.8)		0		0
Total revenue									(0.7)		100.1		17.7
Expenses:
Losses and loss adjustment expenses									0		0		0
Other reinsurance-related expenses									0		0		0
Underwriting, acquisition and insurance expenses									27.1		22.6		27.4
Interest expense									3.9		5.4		4.8
Debt extinguishment costs									0.1		0		0
Foreign currency exchange loss									0		0		0
Total expenses									31.1		28.0		32.2
(Loss) income before income taxes									(31.80)		72.10		(14.50)
Provision (benefit) for income taxes									0		0		0
Net (loss) income before equity in earnings of subsidiaries									(31.8)		72.1		(14.5)
Equity in undistributed earnings of subsidiaries									84.1		(154.0)		101.2
Net (loss) income									52.3		(81.9)		86.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums									434.2		440.7		490.5
Net investment income									86.6		88.9	[1]	89.8	[2]
Fee income, net									1.3		0.1		0.3
Net realized investment and other (losses) gains									19.5		36.9		25.0
Total revenue									541.6		566.6		605.6
Expenses:
Losses and loss adjustment expenses									300.6		290.0		290.9
Other reinsurance-related expenses									0		0		0
Underwriting, acquisition and insurance expenses									169.8		161.5		172.3
Interest expense									17.1		15.5		16.4
Debt extinguishment costs									2.1		0		0
Foreign currency exchange loss									0		0		0.7
Total expenses									489.6		467.0		480.3
(Loss) income before income taxes									52.00		99.60		125.30
Provision (benefit) for income taxes									11.1		28.0		37.5
Net (loss) income before equity in earnings of subsidiaries									40.9		71.6		87.8
Equity in undistributed earnings of subsidiaries									0		0		0
Net (loss) income									40.9		71.6		87.8
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums									752.3	[3]	641.3	[3]	721.1	[3]
Net investment income									32.1	[3]	36.8	[1],[3]	43.6	[2],[3]
Fee income, net									4.0	[3]	1.3	[3]	2.2	[3]
Net realized investment and other (losses) gains									7.0	[3]	12.3	[3]	11.8	[3]
Total revenue									795.4	[3]	691.7	[3]	778.7	[3]
Expenses:
Losses and loss adjustment expenses									447.0	[3]	573.1	[3]	486.6	[3]
Other reinsurance-related expenses									27.3	[3]	5.9	[3]	0	[3]
Underwriting, acquisition and insurance expenses									267.6	[3]	241.6	[3]	266.3	[3]
Interest expense									3.0	[3]	1.2	[3]	1.7	[3]
Debt extinguishment costs									0	[3]	0	[3]	0	[3]
Foreign currency exchange loss									4.3	[3]	3.5	[3]	(4.5)	[3]
Total expenses									749.2	[3]	825.3	[3]	750.1	[3]
(Loss) income before income taxes									46.20	[3]	(133.60)	[3]	28.60	[3]
Provision (benefit) for income taxes									3.3	[3]	(8.0)	[3]	(2.3)	[3]
Net (loss) income before equity in earnings of subsidiaries									42.9	[3]	(125.6)	[3]	30.9	[3]
Equity in undistributed earnings of subsidiaries									0	[3]	0	[3]	0	[3]
Net (loss) income									42.9	[3]	(125.6)	[3]	30.9	[3]
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums									0	[4]	0	[4]	0	[4]
Net investment income									0	[4]	(100.0)	[1],[4]	(17.5)	[2],[4]
Fee income, net									0	[4]	0	[4]	0	[4]
Net realized investment and other (losses) gains									0	[4]	0	[4]	0	[4]
Total revenue									0	[4]	(100.0)	[4]	(17.5)	[4]
Expenses:
Losses and loss adjustment expenses									0	[4]	0	[4]	0	[4]
Other reinsurance-related expenses									0	[4]	0	[4]	0	[4]
Underwriting, acquisition and insurance expenses									0	[4]	0	[4]	0	[4]
Interest expense									(0.3)	[4]	0	[4]	0	[4]
Debt extinguishment costs									0	[4]	0	[4]	0	[4]
Foreign currency exchange loss									0	[4]	0	[4]	0	[4]
Total expenses									(0.3)	[4]	0	[4]	0	[4]
(Loss) income before income taxes									(0.30)	[4]	(100.00)	[4]	(17.50)	[4]
Provision (benefit) for income taxes									0	[4]	0	[4]	0	[4]
Net (loss) income before equity in earnings of subsidiaries									0.3	[4]	(100.0)	[4]	(17.5)	[4]
Equity in undistributed earnings of subsidiaries									(84.1)	[4]	154.0	[4]	(101.2)	[4]
Net (loss) income									$ (83.8)	[4]	$ 54.0	[4]	$ (118.7)	[4]

[1] Included in net investment income for Argo Group parent is intercompany dividend activity of $100.0 million

[2] Included in net investment income for Argo Group parent is intercompany dividend activity of $17.5 million

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

[4] Includes all Argo Group parent company eliminations