Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019		Mar. 31, 2019	Dec. 31, 2018
Assets
Investments	$ 4,811.4	$ 5,099.4	[1]
Cash	158.7	137.8	[1]
Accrued investment income	23.2	25.7	[1]
Premiums receivable	739.1	688.2	[1]
Reinsurance recoverables	2,820.9	3,104.6	[1]
Goodwill and other intangible assets, net	252.9	253.2
Deferred tax asset, net	44.5	6.1	[1]
Deferred acquisition costs, net	158.0	160.2	[1]
Ceded unearned premiums	652.9	545.0	[1]
Operating lease right-of-use assets	89.1	91.8	[1]
Other assets	435.4	387.1	[1]
Assets held for sale	15.2	15.4	[1]
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	0.0	0.0
Total assets	10,201.3	10,514.5	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	5,067.6	5,157.6	[1]	$ 4,668.9
Unearned premiums	1,460.0	1,410.9	[1]
Funds held and ceded reinsurance payable, net	1,154.8	1,253.7
Debt	577.5	578.7
Current income taxes payable, net	17.7	0.8	[1]
Accrued underwriting expenses and other liabilities	185.3	226.0	[1]
Operating lease liabilities	101.6	105.7	[1]
Due to (from) affiliates	0.0	0.0
Total liabilities	8,564.5	8,733.4	[1]
Total shareholders' equity	1,636.8	1,781.1	[1]	$ 1,880.6	$ 1,746.7
Total liabilities and shareholders' equity	10,201.3	10,514.5	[1]
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
Deferred tax asset, net	0.0	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Operating lease right-of-use assets	0.0	0.0
Other assets	0.0	0.0
Assets held for sale	0.0	0.0
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	(1,796.7)	(1,916.7)
Total assets	(1,796.7)	(1,916.7)
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	0.0
Accrued underwriting expenses and other liabilities	0.0	0.0
Operating lease liabilities	0.0	0.0
Due to (from) affiliates	(57.9)	(19.3)
Total liabilities	(57.9)	(19.3)
Total shareholders' equity	(1,738.8)	(1,897.4)
Total liabilities and shareholders' equity	(1,796.7)	(1,916.7)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,487.8	1,693.2
Cash	116.5	104.3
Accrued investment income	6.5	7.5
Premiums receivable	474.7	456.9
Reinsurance recoverables	1,096.0	1,415.2
Goodwill and other intangible assets, net	89.2	89.2
Deferred tax asset, net	4.4	5.7
Deferred acquisition costs, net	71.7	71.8
Ceded unearned premiums	316.7	238.6
Operating lease right-of-use assets	24.5	25.1
Other assets	264.2	213.5
Assets held for sale	0.0	0.0
Intercompany note receivable	(57.4)	(56.7)
Investments in subsidiaries	0.0	0.0
Total assets	3,894.8	4,264.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,939.6	2,120.1
Unearned premiums	562.9	511.1
Funds held and ceded reinsurance payable, net	463.1	607.8
Debt	139.8	141.0
Current income taxes payable, net	(5.4)	(7.4)
Accrued underwriting expenses and other liabilities	101.9	124.8
Operating lease liabilities	27.8	29.5
Due to (from) affiliates	25.6	13.4
Total liabilities	3,255.3	3,540.3
Total shareholders' equity	639.5	724.0
Total liabilities and shareholders' equity	3,894.8	4,264.3
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	0.3	0.6
Cash	1.8	1.9
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	40.3	40.6
Deferred tax asset, net	0.0	0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Operating lease right-of-use assets	7.1	7.1
Other assets	18.4	7.8
Assets held for sale	0.0	0.0
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	1,796.7	1,916.7
Total assets	1,864.6	1,974.7
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
Current income taxes payable, net	0.0	0.0
Accrued underwriting expenses and other liabilities	9.2	13.6
Operating lease liabilities	7.3	7.3
Due to (from) affiliates	57.9	19.3
Total liabilities	227.8	193.6
Total shareholders' equity	1,636.8	1,781.1
Total liabilities and shareholders' equity	1,864.6	1,974.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,323.3	3,405.6
Cash	40.4	31.6
Accrued investment income	16.7	18.2
Premiums receivable	264.4	231.3
Reinsurance recoverables	1,724.9	1,689.4
Goodwill and other intangible assets, net	123.4	123.4
Deferred tax asset, net	40.1	0.4
Deferred acquisition costs, net	86.3	88.4
Ceded unearned premiums	336.2	306.4
Operating lease right-of-use assets	57.5	59.6
Other assets	152.8	165.8
Assets held for sale	15.2	15.4
Intercompany note receivable	57.4	56.7
Investments in subsidiaries	0.0	0.0
Total assets	6,238.6	6,192.2
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,128.0	3,037.5
Unearned premiums	897.1	899.8
Funds held and ceded reinsurance payable, net	691.7	645.9
Debt	284.3	284.3
Current income taxes payable, net	23.1	8.2
Accrued underwriting expenses and other liabilities	74.2	87.6
Operating lease liabilities	66.5	68.9
Due to (from) affiliates	(25.6)	(13.4)
Total liabilities	5,139.3	5,018.8
Total shareholders' equity	1,099.3	1,173.4
Total liabilities and shareholders' equity	$ 6,238.6	$ 6,192.2

[1] Derived from audited consolidated financial statements.