Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Investments	$ 5,099.4	$ 4,787.0
Cash	137.8	139.2
Accrued investment income	25.7	27.2
Premiums receivable	688.2	649.9
Reinsurance recoverables	3,104.6	2,688.3
Goodwill and other intangible assets, net	253.2	270.5
Current income taxes receivable, net	0.0	8.2
Deferred tax asset, net	6.1	0.0
Deferred acquisition costs, net	160.2	167.3
Ceded unearned premiums	545.0	457.7
Operating leases right-of-use assets	91.8
Other assets	387.1	362.9
Assets held for sale	15.4	0.0
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	0.0	0.0
Total assets	10,514.5	9,558.2
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	5,157.6	4,654.6	$ 4,201.0
Unearned premiums	1,410.9	1,300.9
Funds held and ceded reinsurance payable, net	1,253.7	1,007.7
Debt	578.7	580.2
Current income taxes payable, net	0.8	0.0
Deferred tax liabilities, net	0.0	6.2
Accrued underwriting expenses and other liabilities	226.0	261.9
Operating lease liabilities	105.7
Due to (from) affiliates	0.0	0.0
Intercompany note payable		0.0
Total liabilities	8,733.4	7,811.5
Total shareholders' equity	1,781.1	1,746.7	1,819.7	$ 1,792.7
Total liabilities and shareholders' equity	10,514.5	9,558.2
Consolidating Adjustments [Member]
Assets
Investments	0.0	0.0
Cash	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 tax asset, net	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Operating leases right-of-use assets	0.0
Other assets	0.0	0.0
Assets held for sale	0.0
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	(1,916.7)	(1,852.7)
Total assets	(1,916.7)	(1,852.7)
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	0.0	0.0
Current income taxes payable, net	0.0
Deferred tax liabilities, net		0.0
Accrued underwriting expenses and other liabilities	0.0	0.0
Operating lease liabilities	0.0
Due to (from) affiliates	(19.3)	(8.5)
Intercompany note payable		0.0
Total liabilities	(19.3)	(8.5)
Total shareholders' equity	(1,897.4)	(1,844.2)
Total liabilities and shareholders' equity	(1,916.7)	(1,852.7)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,693.2	1,607.3
Cash	104.3	105.8
Accrued investment income	7.5	6.9
Premiums receivable	456.9	420.4
Reinsurance recoverables	1,415.2	1,053.1
Goodwill and other intangible assets, net	89.2	104.8
Current income taxes receivable, net		(0.9)
Deferred tax asset, net	5.7
Deferred acquisition costs, net	71.8	81.1
Ceded unearned premiums	238.6	207.3
Operating leases right-of-use assets	25.1
Other assets	213.5	181.9
Assets held for sale	0.0
Intercompany note receivable	(56.7)	(53.7)
Investment in subsidiaries	0.0	0.0
Total assets	4,264.3	3,714.0
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,120.1	1,883.2
Unearned premiums	511.1	503.5
Funds held and ceded reinsurance payable, net	607.8	268.4
Debt	141.0	142.1
Current income taxes payable, net	(7.4)
Deferred tax liabilities, net		0.6
Accrued underwriting expenses and other liabilities	124.8	142.3
Operating lease liabilities	29.5
Due to (from) affiliates	13.4	(2.0)
Intercompany note payable		(19.1)
Total liabilities	3,540.3	2,919.0
Total shareholders' equity	724.0	795.0
Total liabilities and shareholders' equity	4,264.3	3,714.0
Argo Group [Member]
Assets
Investments	0.6	3.8
Cash	1.9	1.7	$ 0.9	$ 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	40.6	41.9
Current income taxes receivable, net		0.0
Deferred tax asset, net	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Operating leases right-of-use assets	7.1
Other assets	7.8	15.7
Assets held for sale	0.0
Intercompany note receivable	0.0	0.0
Investment in subsidiaries	1,916.7	1,852.7
Total assets	1,974.7	1,915.8
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
Current income taxes payable, net	0.0
Deferred tax liabilities, net		0.0
Accrued underwriting expenses and other liabilities	13.6	7.2
Operating lease liabilities	7.3
Due to (from) affiliates	19.3	8.5
Intercompany note payable		0.0
Total liabilities	193.6	169.1
Total shareholders' equity	1,781.1	1,746.7
Total liabilities and shareholders' equity	1,974.7	1,915.8
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,405.6	3,175.9
Cash	31.6	31.7
Accrued investment income	18.2	20.3
Premiums receivable	231.3	229.5
Reinsurance recoverables	1,689.4	1,635.2
Goodwill and other intangible assets, net	123.4	123.8
Current income taxes receivable, net		9.1
Deferred tax asset, net	0.4
Deferred acquisition costs, net	88.4	86.2
Ceded unearned premiums	306.4	250.4
Operating leases right-of-use assets	59.6
Other assets	165.8	165.3
Assets held for sale	15.4
Intercompany note receivable	56.7	53.7
Investment in subsidiaries	0.0	0.0
Total assets	6,192.2	5,781.1
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,037.5	2,771.4
Unearned premiums	899.8	797.4
Funds held and ceded reinsurance payable, net	645.9	739.3
Debt	284.3	284.7
Current income taxes payable, net	8.2
Deferred tax liabilities, net		5.6
Accrued underwriting expenses and other liabilities	87.6	112.4
Operating lease liabilities	68.9
Due to (from) affiliates	(13.4)	2.0
Intercompany note payable		19.1
Total liabilities	5,018.8	4,731.9
Total shareholders' equity	1,173.4	1,049.2
Total liabilities and shareholders' equity	$ 6,192.2	$ 5,781.1