Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Assets
Investments		$ 5,006.0		$ 5,099.4	[1]
Cash		300.4		137.8	[1]
Accrued investment income		22.2		25.7	[1]
Premiums receivable		797.0		688.2	[1]
Reinsurance recoverables		3,046.5		3,104.6	[1]
Goodwill and other intangible assets, net		247.3		253.2
Current income taxes receivable, net	$ 11.2			0.0	[1]
Deferred tax asset, net		17.0		6.1	[1]
Deferred acquisition costs, net		170.4		160.2	[1]
Ceded unearned premiums		660.9		545.0	[1]
Operating lease right-of-use assets		84.4		91.8	[1]
Other assets		358.5		387.1	[1]
Assets held for sale		12.5		15.4	[1]
Intercompany note receivable		0.0		0.0
Investments in subsidiaries		0.0		0.0
Total assets		10,734.3		10,514.5	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		5,390.0		5,157.6	[1]	$ 4,842.5
Unearned premiums		1,574.2		1,410.9	[1]
Funds held and ceded reinsurance payable, net		1,147.6		1,253.7
Debt		456.9		578.7
Current income taxes payable, net		0.0		0.8	[1]
Accrued underwriting expenses and other liabilities		202.8		226.0	[1]
Operating lease liabilities		98.3		105.7	[1]
Due to (from) affiliates		0.0		0.0
Total liabilities		8,869.8		8,733.4	[1]
Total shareholders' equity		1,864.5	$ 1,730.0	1,781.1	[1]	$ 1,893.4	$ 1,929.0	$ 1,746.7
Total liabilities and shareholders' equity		10,734.3		10,514.5	[1]
Consolidating Adjustments
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		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		(1,932.2)		(1,916.7)
Total assets		(1,932.2)		(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
Accrued underwriting expenses and other liabilities		0.0		0.0
Operating lease liabilities		0.0		0.0
Due to (from) affiliates		(87.7)		(19.3)
Total liabilities		(87.7)		(19.3)
Total shareholders' equity		(1,844.5)		(1,897.4)
Total liabilities and shareholders' equity		(1,932.2)		(1,916.7)
Other Subsidiaries and Eliminations
Assets
Investments		1,625.4		1,693.2
Cash		163.9		104.3
Accrued investment income		5.2		7.5
Premiums receivable		491.9		456.9
Reinsurance recoverables		1,229.2		1,415.2
Goodwill and other intangible assets, net		89.2		89.2
Current income taxes receivable, net	8.5
Deferred tax asset, net		12.9		5.7
Deferred acquisition costs, net		72.2		71.8
Ceded unearned premiums		318.9		238.6
Operating lease right-of-use assets		24.6		25.1
Other assets		191.8		213.5
Assets held for sale		0.3		0.0
Intercompany note receivable		(58.4)		(56.7)
Investments in subsidiaries		0.0		0.0
Total assets		4,175.6		4,264.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		2,104.5		2,120.1
Unearned premiums		618.2		511.1
Funds held and ceded reinsurance payable, net		571.4		607.8
Debt		144.1		141.0
Current income taxes payable, net				(7.4)
Accrued underwriting expenses and other liabilities		114.7		124.8
Operating lease liabilities		29.8		29.5
Due to (from) affiliates		14.2		13.4
Total liabilities		3,596.9		3,540.3
Total shareholders' equity		578.7		724.0
Total liabilities and shareholders' equity		4,175.6		4,264.3
Argo Group International Holdings, Ltd. (Parent Guarantor)
Assets
Investments		1.2		0.6
Cash		2.9		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		39.6		40.6
Current income taxes receivable, net	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		6.5		7.1
Other assets		8.8		7.8
Assets held for sale		0.0		0.0
Intercompany note receivable		0.0		0.0
Investments in subsidiaries		1,932.2		1,916.7
Total assets		1,991.2		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		28.4		153.4
Current income taxes payable, net				0.0
Accrued underwriting expenses and other liabilities		4.0		13.6
Operating lease liabilities		6.6		7.3
Due to (from) affiliates		87.7		19.3
Total liabilities		126.7		193.6
Total shareholders' equity		1,864.5		1,781.1
Total liabilities and shareholders' equity		1,991.2		1,974.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Assets
Investments		3,379.4		3,405.6
Cash		133.6		31.6
Accrued investment income		17.0		18.2
Premiums receivable		305.1		231.3
Reinsurance recoverables		1,817.3		1,689.4
Goodwill and other intangible assets, net		118.5		123.4
Current income taxes receivable, net	$ 2.7
Deferred tax asset, net		4.1		0.4
Deferred acquisition costs, net		98.2		88.4
Ceded unearned premiums		342.0		306.4
Operating lease right-of-use assets		53.3		59.6
Other assets		157.9		165.8
Assets held for sale		12.2		15.4
Intercompany note receivable		58.4		56.7
Investments in subsidiaries		0.0		0.0
Total assets		6,499.7		6,192.2
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses		3,285.5		3,037.5
Unearned premiums		956.0		899.8
Funds held and ceded reinsurance payable, net		576.2		645.9
Debt		284.4		284.3
Current income taxes payable, net				8.2
Accrued underwriting expenses and other liabilities		84.1		87.6
Operating lease liabilities		61.9		68.9
Due to (from) affiliates		(14.2)		(13.4)
Total liabilities		5,233.9		5,018.8
Total shareholders' equity		1,265.8		1,173.4
Total liabilities and shareholders' equity		$ 6,499.7		$ 6,192.2

[1] Derived from audited consolidated financial statements.