Information Provided in Connection with Outstanding Debt of Subsidiaries - Condensed Consolidating Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014		Dec. 31, 2013
Assets
Investments	$ 4,064.4		$ 4,097.9	[1]
Cash	89.1		81.0	[1]	149.5		157.4
Accrued investment income	20.9		22.1	[1]
Premiums receivable	371.7		353.6	[1]
Reinsurance recoverables	987.3		997.2	[1]
Goodwill and other intangible assets, net	231.0		230.8
Current income taxes receivable, net	4.2		14.9	[1]
Deferred acquisition costs, net	129.7		124.6	[1]
Ceded unearned premiums	256.6		207.6	[1]
Other assets	222.2		226.6	[1]
Due from (to) affiliates	0		0
Intercompany note receivable	0		0
Investments in subsidiaries	0		0
Total assets	6,377.1		6,356.3	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,035.1		3,042.4	[1]	3,042.4
Unearned premiums	826.4		817.2	[1]
Funds held and ceded reinsurance payable, net	260.5		233.8
Long-term debt	374.0		378.5
Current income taxes payable, net	0
Deferred tax liabilities, net	53.8		53.0	[1]
Accrued underwriting expenses and other liabilities	164.5		184.7
Total liabilities	4,714.3		4,709.6	[1]
Total shareholders' equity	1,662.8		1,646.7	[1]
Total liabilities and shareholders' equity	6,377.1		6,356.3	[1]
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	7.2		0.7
Cash	0		0		0		0
Accrued investment income	0		0
Premiums receivable	0		0
Reinsurance recoverables	0		0
Goodwill and other intangible assets, net	0		0
Current income taxes receivable, net	0		0
Deferred acquisition costs, net	0		0
Ceded unearned premiums	0		0
Other assets	12.7		9.6
Due from (to) affiliates	(16.0)		2.9
Intercompany note receivable	0		0
Investments in subsidiaries	1,722.8		1,698.0
Total assets	1,726.7		1,711.2
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	49.0		49.0
Current income taxes payable, net	0
Deferred tax liabilities, net	0		0
Accrued underwriting expenses and other liabilities	14.9		15.5
Total liabilities	63.9		64.5
Total shareholders' equity	1,662.8		1,646.7
Total liabilities and shareholders' equity	1,726.7		1,711.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	2,851.4		2,841.5
Cash	54.8		49.3		118.4		132.1
Accrued investment income	17.1		17.8
Premiums receivable	160.0		154.6
Reinsurance recoverables	1,152.0		1,173.6
Goodwill and other intangible assets, net	132.7		131.7
Current income taxes receivable, net	0		10.1
Deferred acquisition costs, net	60.0		58.0
Ceded unearned premiums	112.2		98.5
Other assets	175.4		174.1
Due from (to) affiliates	(19.3)		(19.8)
Intercompany note receivable	55.6		72.0
Investments in subsidiaries	0		0
Total assets	4,751.9		4,761.4
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,138.8		2,136.4
Unearned premiums	458.8		448.9
Funds held and ceded reinsurance payable, net	662.8		675.1
Long-term debt	288.7		288.7
Current income taxes payable, net	0.5
Deferred tax liabilities, net	44.7		41.2
Accrued underwriting expenses and other liabilities	91.7		104.2
Total liabilities	3,686.0		3,694.5
Total shareholders' equity	1,065.9		1,066.9
Total liabilities and shareholders' equity	4,751.9		4,761.4
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,205.8	[2]	1,255.7	[2]
Cash	34.3	[2]	31.7	[2]	31.1	[2]	25.3	[2]
Accrued investment income	3.8	[2]	4.3	[2]
Premiums receivable	211.7	[2]	199.0	[2]
Reinsurance recoverables	(164.7)	[2]	(176.4)	[2]
Goodwill and other intangible assets, net	98.3	[2]	99.1	[2]
Current income taxes receivable, net	4.2	[2]	4.8	[2]
Deferred acquisition costs, net	69.7	[2]	66.6	[2]
Ceded unearned premiums	144.4	[2]	109.1	[2]
Other assets	59.1	[2]	67.9	[2]
Due from (to) affiliates	19.3	[2]	19.8	[2]
Intercompany note receivable	(55.6)	[2]	(72.0)	[2]
Investments in subsidiaries	0	[2]	0	[2]
Total assets	1,630.3	[2]	1,609.6	[2]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	896.3	[2]	906.0	[2]
Unearned premiums	367.6	[2]	368.3	[2]
Funds held and ceded reinsurance payable, net	(402.3)	[2]	(441.3)	[2]
Long-term debt	56.9	[2]	61.4	[2]
Current income taxes payable, net	(0.5)	[2]
Deferred tax liabilities, net	9.1	[2]	11.8	[2]
Accrued underwriting expenses and other liabilities	57.9	[2]	65.0	[2]
Total liabilities	985.0	[2]	971.2	[2]
Total shareholders' equity	645.3	[2]	638.4	[2]
Total liabilities and shareholders' equity	1,630.3	[2]	1,609.6	[2]
Consolidating Adjustments [Member]
Assets
Investments	0	[3]	0	[3]
Cash	0	[3]	0	[3]	0	[3]	0	[3]
Accrued investment income	0	[3]	0	[3]
Premiums receivable	0	[3]	0	[3]
Reinsurance recoverables	0	[3]	0	[3]
Goodwill and other intangible assets, net	0	[3]	0	[3]
Current income taxes receivable, net	0	[3]	0	[3]
Deferred acquisition costs, net	0	[3]	0	[3]
Ceded unearned premiums	0	[3]	0	[3]
Other assets	(25.0)	[3]	(25.0)	[3]
Due from (to) affiliates	16.0	[3]	(2.9)	[3]
Intercompany note receivable	0	[3]	0	[3]
Investments in subsidiaries	(1,722.8)	[3]	(1,698.0)	[3]
Total assets	(1,731.8)	[3]	(1,725.9)	[3]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	0	[3]	0	[3]
Unearned premiums	0	[3]	0	[3]
Funds held and ceded reinsurance payable, net	0	[3]	0	[3]
Long-term debt	(20.6)	[3]	(20.6)	[3]
Current income taxes payable, net	0	[3]
Deferred tax liabilities, net	0	[3]	0	[3]
Accrued underwriting expenses and other liabilities	0	[3]	0	[3]
Total liabilities	(20.6)	[3]	(20.6)	[3]
Total shareholders' equity	(1,711.2)	[3]	(1,705.3)	[3]
Total liabilities and shareholders' equity	$ (1,731.8)	[3]	$ (1,725.9)	[3]

[1] Derived from audited consolidated financial statements.

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

[3] Includes all Argo Group parent company eliminations.