Information Provided In Connection With Outstanding Debt Of Subsidiaries (Condensed Consolidating Balance Sheet) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011	Dec. 31, 2010
Assets
Investments	$ 4,344.9		$ 4,147.5
Cash	142.7		100.9		119.8	83.5
Premiums receivable	402.9		309.0
Reinsurance recoverables	1,016.5		1,144.1
Goodwill and other intangible assets	242.2		246.8
Current income taxes receivable, net	21.3		11.2
Deferred acquisition costs, net	107.7		101.5
Ceded unearned premiums	213.0		179.4
Other assets	129.8		105.6
Due from affiliates	0		0
Intercompany note receivable	0		0
Investment in subsidiaries	0		0
Total assets	6,649.8		6,378.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,227.9		3,291.1
Unearned premiums	787.0		658.2
Funds held and ceded reinsurance payable, net	397.7		460.3
Long-term debt	519.8		377.0
Current income taxes payable, net			0
Deferred tax liabilities, net	49.7		18.5
Accrued underwriting expenses and other liabilities	29.6		31.7
Due to affiliates	0		0
Intercompany note payable	0
Total liabilities	5,107.9		4,915.3
Total shareholders' equity	1,541.9		1,463.0
Total liabilities and shareholders' equity	6,649.8		6,378.3
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	3.0		(0.8)
Cash	0.1		0		0	0
Premiums receivable	0		0
Reinsurance recoverables	0		0
Goodwill and other intangible assets	0		0
Current income taxes receivable, net	0		0
Deferred acquisition costs, net	0		0
Ceded unearned premiums	0		0
Other assets	4.4		6.8
Due from affiliates	0		0
Intercompany note receivable	0		0
Investment in subsidiaries	1,640.2		1,530.6
Total assets	1,647.7		1,536.6
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Funds held and ceded reinsurance payable, net	0		0
Long-term debt	64.4		64.4
Current income taxes payable, net			0
Deferred tax liabilities, net	0		0
Accrued underwriting expenses and other liabilities	10.6		8.9
Due to affiliates	3.6		0.3
Intercompany note payable	27.2
Total liabilities	105.8		73.6
Total shareholders' equity	1,541.9		1,463.0
Total liabilities and shareholders' equity	1,647.7		1,536.6
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	2,925.7		2,778.5
Cash	127.8		85.1		79.1	61.6
Premiums receivable	153.2		151.2
Reinsurance recoverables	1,143.3		1,140.5
Goodwill and other intangible assets	136.1		137.2
Current income taxes receivable, net	7.1		0
Deferred acquisition costs, net	57.3		55.8
Ceded unearned premiums	88.9		77.2
Other assets	106.2		88.5
Due from affiliates	0.7		0
Intercompany note receivable	102.9		90.8
Investment in subsidiaries	0		0
Total assets	4,849.2		4,604.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,238.4		2,291.2
Unearned premiums	432.0		397.8
Funds held and ceded reinsurance payable, net	579.8		562.8
Long-term debt	391.6		247.9
Current income taxes payable, net			1.8
Deferred tax liabilities, net	42.8		13.4
Accrued underwriting expenses and other liabilities	81.0		81.7
Due to affiliates	0		0.6
Intercompany note payable	25.0
Total liabilities	3,790.6		3,597.2
Total shareholders' equity	1,058.6		1,007.6
Total liabilities and shareholders' equity	4,849.2		4,604.8
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,416.2	[1]	1,369.8	[1]
Cash	14.8	[1]	15.8	[1]	40.7	21.9
Premiums receivable	249.7	[1]	157.8	[1]
Reinsurance recoverables	(126.8)	[1]	3.6	[1]
Goodwill and other intangible assets	106.1	[1]	109.6	[1]
Current income taxes receivable, net	14.2	[1]	11.2	[1]
Deferred acquisition costs, net	50.4	[1]	45.7	[1]
Ceded unearned premiums	124.1	[1]	102.2	[1]
Other assets	48.0	[1]	42.6	[1]
Due from affiliates	(0.7)	[1]	0.3	[1]
Intercompany note receivable	(102.9)	[1]	(90.8)	[1]
Investment in subsidiaries	0	[1]	0	[1]
Total assets	1,793.1	[1]	1,767.8	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	989.5	[1]	999.9	[1]
Unearned premiums	355.0	[1]	260.4	[1]
Funds held and ceded reinsurance payable, net	(182.1)	[1]	(102.5)	[1]
Long-term debt	63.8	[1]	64.7	[1]
Current income taxes payable, net			(1.8)	[1]
Deferred tax liabilities, net	6.9	[1]	5.1	[1]
Accrued underwriting expenses and other liabilities	34.2	[1]	19.6	[1]
Due to affiliates	0	[1]	(0.6)	[1]
Intercompany note payable	(25.0)	[1]
Total liabilities	1,242.3	[1]	1,244.8	[1]
Total shareholders' equity	550.8	[1]	523.0	[1]
Total liabilities and shareholders' equity	1,793.1	[1]	1,767.8	[1]
Consolidating Adjustments [Member]
Assets
Investments	0	[2]	0	[2]
Cash	0	[2]	0	[2]	0	0
Premiums receivable	0	[2]	0	[2]
Reinsurance recoverables	0	[2]	0	[2]
Goodwill and other intangible assets	0	[2]	0	[2]
Current income taxes receivable, net	0	[2]	0	[2]
Deferred acquisition costs, net	0	[2]	0	[2]
Ceded unearned premiums	0	[2]	0	[2]
Other assets	0	[2]	0	[2]
Due from affiliates	0	[2]	(0.3)	[2]
Intercompany note receivable	0	[2]	0	[2]
Investment in subsidiaries	(1,640.2)	[2]	(1,530.6)	[2]
Total assets	(1,640.2)	[2]	(1,530.9)	[2]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	0	[2]	0	[2]
Unearned premiums	0	[2]	0	[2]
Funds held and ceded reinsurance payable, net	0	[2]	0	[2]
Long-term debt	0	[2]	0	[2]
Current income taxes payable, net			0	[2]
Deferred tax liabilities, net	0	[2]	0	[2]
Accrued underwriting expenses and other liabilities	0	[2]	0	[2]
Due to affiliates	(3.6)	[2]	(0.3)	[2]
Intercompany note payable	(27.2)	[2]
Total liabilities	(30.8)	[2]	(0.3)	[2]
Total shareholders' equity	(1,609.4)	[2]	(1,530.6)	[2]
Total liabilities and shareholders' equity	$ (1,640.2)	[2]	$ (1,530.9)	[2]

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

[2] Includes all Argo Group parent company eliminations