Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Investments	$ 4,787.0	$ 4,742.9
Cash	139.2	176.6	$ 86.0	$ 121.7
Accrued investment income	27.2	23.5
Premiums receivable	649.9	598.6
Reinsurance recoverables	2,688.3	2,093.3
Goodwill and other intangible assets, net	270.5	258.2
Current income taxes receivable, net	8.2	1.4
Deferred acquisition costs, net	167.3	160.4
Ceded unearned premiums	457.7	399.5
Other assets	362.9	309.6
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	0.0	0.0
Total assets	9,558.2	8,764.0
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,654.6	4,201.0	3,350.8
Unearned premiums	1,300.9	1,207.7
Funds held and ceded reinsurance payable, net	1,007.7	776.7
Debt	580.2	580.7
Deferred tax liabilities, net	6.2	31.3
Accrued underwriting expenses and other liabilities	261.9	146.9
Due to (from) affiliates	0.0	0.0
Intercompany note payable	0.0
Total liabilities	7,811.5	6,944.3
Total shareholders' equity	1,746.7	1,819.7	1,792.7	1,668.1
Total liabilities and shareholders' equity	9,558.2	8,764.0
Consolidating Adjustments [Member]
Assets
Investments	0.0	0.0
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	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Other assets	0.0	0.0
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	(1,852.7)	(1,940.0)
Total assets	(1,852.7)	(1,940.0)
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
Debt	0.0	0.0
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	0.0	0.0
Due to (from) affiliates	(8.5)	(11.7)
Intercompany note payable	0.0
Total liabilities	(8.5)	(11.7)
Total shareholders' equity	(1,844.2)	(1,928.3)
Total liabilities and shareholders' equity	(1,852.7)	(1,940.0)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,607.3	1,663.5
Cash	105.8	127.9	32.3	32.9
Accrued investment income	6.9	5.5
Premiums receivable	420.4	382.1
Reinsurance recoverables	1,053.1	606.0
Goodwill and other intangible assets, net	104.8	90.1
Current income taxes receivable, net	(0.9)	(1.0)
Deferred acquisition costs, net	81.1	79.7
Ceded unearned premiums	207.3	201.0
Other assets	181.9	128.9
Intercompany note receivable	(53.7)	(50.9)
Investment in subsidiaries	0.0	0.0
Total assets	3,714.0	3,232.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,883.2	1,717.1
Unearned premiums	503.5	503.7
Funds held and ceded reinsurance payable, net	268.4	(22.7)
Debt	142.1	142.8
Deferred tax liabilities, net	0.6	(1.2)
Accrued underwriting expenses and other liabilities	142.3	43.0
Due to (from) affiliates	(2.0)	0.4
Intercompany note payable	(19.1)
Total liabilities	2,919.0	2,383.1
Total shareholders' equity	795.0	849.7
Total liabilities and shareholders' equity	3,714.0	3,232.8
Argo Group [Member]
Assets
Investments	3.8	0.4
Cash	1.7	0.9	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	41.9	43.2
Current income taxes receivable, net	0.0	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Other assets	15.7	9.2
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	1,852.7	1,940.0
Total assets	1,915.8	1,993.7
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
Debt	153.4	153.4
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	7.2	8.9
Due to (from) affiliates	8.5	11.7
Intercompany note payable	0.0
Total liabilities	169.1	174.0
Total shareholders' equity	1,746.7	1,819.7
Total liabilities and shareholders' equity	1,915.8	1,993.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,175.9	3,079.0
Cash	31.7	47.8	$ 53.7	$ 88.8
Accrued investment income	20.3	18.0
Premiums receivable	229.5	216.5
Reinsurance recoverables	1,635.2	1,487.3
Goodwill and other intangible assets, net	123.8	124.9
Current income taxes receivable, net	9.1	2.4
Deferred acquisition costs, net	86.2	80.7
Ceded unearned premiums	250.4	198.5
Other assets	165.3	171.5
Intercompany note receivable	53.7	50.9
Investment in subsidiaries	0.0	0.0
Total assets	5,781.1	5,477.5
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,771.4	2,483.9
Unearned premiums	797.4	704.0
Funds held and ceded reinsurance payable, net	739.3	799.4
Debt	284.7	284.5
Deferred tax liabilities, net	5.6	32.5
Accrued underwriting expenses and other liabilities	112.4	95.0
Due to (from) affiliates	2.0	(0.4)
Intercompany note payable	19.1
Total liabilities	4,731.9	4,398.9
Total shareholders' equity	1,049.2	1,078.6
Total liabilities and shareholders' equity	$ 5,781.1	$ 5,477.5