Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Investments		$ 4,324.3	$ 4,115.7
Cash		86.0	121.7	$ 81.0	$ 157.4
Accrued investment income		20.7	21.6
Premiums receivable		463.8	404.5
Reinsurance recoverables		1,385.6	1,121.1
Goodwill and other intangible assets, net		219.9	225.5
Current income taxes receivable, net		0.0	11.6
Deferred acquisition costs, net		139.1	132.4
Ceded unearned premiums		302.8	250.8
Other assets		262.8	220.7
Intercompany note receivable		0.0	0.0
Investments in subsidiaries		0.0	0.0
Total assets		7,205.0	6,625.6
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		3,350.8	3,123.6	3,042.4
Unearned premiums		970.0	886.7
Funds held and ceded reinsurance payable, net		543.7	390.0
Long-term debt		367.6	367.2
Current income taxes payable, net		8.1	0.0
Deferred tax liabilities, net		24.1	23.6
Accrued underwriting expenses and other liabilities		148.0	166.4
Due to affiliates		0.0	0.0
Total liabilities		5,412.3	4,957.5
Total shareholders' equity		1,792.7	1,668.1	1,646.7	1,563.0
Total liabilities and shareholders' equity		7,205.0	6,625.6
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments		2.2	6.2
Cash		0.0	0.0	0.0	0.0
Accrued investment income		0.0	0.0
Premiums receivable		0.0	0.0
Reinsurance recoverables		0.0	0.0
Goodwill and other intangible assets, net		0.0	0.0
Current income taxes receivable, net			0.0
Deferred acquisition costs, net		0.0	0.0
Ceded unearned premiums		0.0	0.0
Other assets		8.7	8.2
Intercompany note receivable		0.0	0.0
Investments in subsidiaries		1,834.4	1,715.9
Total assets		1,845.3	1,730.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		0.0	0.0
Unearned premiums		0.0	0.0
Funds held and ceded reinsurance payable, net		0.0	0.0
Long-term debt		28.4	28.4
Current income taxes payable, net		0.0
Deferred tax liabilities, net		0.0	0.0
Accrued underwriting expenses and other liabilities		13.7	16.3
Due to affiliates		10.5	17.5
Total liabilities		52.6	62.2
Total shareholders' equity		1,792.7	1,668.1
Total liabilities and shareholders' equity		1,845.3	1,730.3
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments		2,834.2	2,761.0
Cash		53.7	88.8	49.3	132.1
Accrued investment income		16.0	16.4
Premiums receivable		204.9	166.4
Reinsurance recoverables		1,348.4	1,212.2
Goodwill and other intangible assets, net		127.1	129.8
Current income taxes receivable, net			4.7
Deferred acquisition costs, net		63.5	58.2
Ceded unearned premiums		168.9	125.8
Other assets		168.0	151.7
Intercompany note receivable		50.2	49.8
Investments in subsidiaries		0.0	0.0
Total assets		5,034.9	4,764.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		2,322.4	2,194.1
Unearned premiums		580.0	501.5
Funds held and ceded reinsurance payable, net		750.2	702.6
Long-term debt		284.4	284.2
Current income taxes payable, net		8.5
Deferred tax liabilities, net		17.6	11.9
Accrued underwriting expenses and other liabilities		92.0	95.4
Due to affiliates		1.8	2.3
Total liabilities		4,056.9	3,792.0
Total shareholders' equity		978.0	972.8
Total liabilities and shareholders' equity		5,034.9	4,764.8
Other Subsidiaries and Eliminations [Member]
Assets
Investments	[1]	1,487.9	1,348.5
Cash	[1]	32.3	32.9	31.7	25.3
Accrued investment income	[1]	4.7	5.2
Premiums receivable	[1]	258.9	238.1
Reinsurance recoverables	[1]	37.2	(91.1)
Goodwill and other intangible assets, net	[1]	92.8	95.7
Current income taxes receivable, net	[1]		6.9
Deferred acquisition costs, net	[1]	75.6	74.2
Ceded unearned premiums	[1]	133.9	125.0
Other assets	[1]	86.1	60.8
Intercompany note receivable	[1]	(50.2)	(49.8)
Investments in subsidiaries	[1]	0.0	0.0
Total assets	[1]	2,159.2	1,846.4
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[1]	1,028.4	929.5
Unearned premiums	[1]	390.0	385.2
Funds held and ceded reinsurance payable, net	[1]	(206.5)	(312.6)
Long-term debt	[1]	54.8	54.6
Current income taxes payable, net	[1]	(0.4)
Deferred tax liabilities, net	[1]	6.5	11.7
Accrued underwriting expenses and other liabilities	[1]	42.3	54.7
Due to affiliates	[1]	(1.8)	(2.3)
Total liabilities	[1]	1,313.3	1,120.8
Total shareholders' equity	[1]	845.9	725.6
Total liabilities and shareholders' equity	[1]	2,159.2	1,846.4
Consolidating Adjustments [Member]
Assets
Investments	[2]	0.0	0.0
Cash	[2]	0.0	0.0	$ 0.0	$ 0.0
Accrued investment income	[2]	0.0	0.0
Premiums receivable	[2]	0.0	0.0
Reinsurance recoverables	[2]	0.0	0.0
Goodwill and other intangible assets, net	[2]	0.0	0.0
Current income taxes receivable, net	[2]		0.0
Deferred acquisition costs, net	[2]	0.0	0.0
Ceded unearned premiums	[2]	0.0	0.0
Other assets	[2]	0.0	0.0
Intercompany note receivable	[2]	0.0	0.0
Investments in subsidiaries	[2]	(1,834.4)	(1,715.9)
Total assets	[2]	(1,834.4)	(1,715.9)
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	[2]	0.0	0.0
Unearned premiums	[2]	0.0	0.0
Funds held and ceded reinsurance payable, net	[2]	0.0	0.0
Long-term debt	[2]	0.0	0.0
Current income taxes payable, net	[2]	0.0
Deferred tax liabilities, net	[2]	0.0	0.0
Accrued underwriting expenses and other liabilities	[2]	0.0	0.0
Due to affiliates	[2]	(10.5)	(17.5)
Total liabilities	[2]	(10.5)	(17.5)
Total shareholders' equity	[2]	(1,823.9)	(1,698.4)
Total liabilities and shareholders' equity	[2]	$ (1,834.4)	$ (1,715.9)

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

[2] Includes all Argo Group parent company eliminations.