Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Investments	$ 4,851.5		$ 5,099.4	[1]
Cash	187.7		137.8	[1]
Accrued investment income	23.4		25.7	[1]
Premiums receivable	800.2		688.2	[1]
Reinsurance recoverables	2,942.8		3,104.6	[1]
Goodwill and other intangible assets, net	247.6		253.2
Deferred tax asset, net	18.9		6.1	[1]
Deferred acquisition costs, net	153.5		160.2	[1]
Ceded unearned premiums	656.5		545.0	[1]
Operating lease right-of-use assets	87.2		91.8	[1]
Other assets	414.7		387.1	[1]
Assets held for sale	12.7		15.4	[1]
Intercompany note receivable	0.0		0.0
Investments in subsidiaries	0.0		0.0
Total assets	10,396.7		10,514.5	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	5,216.3		5,157.6	[1]	$ 4,735.7
Unearned premiums	1,481.2		1,410.9	[1]
Funds held and ceded reinsurance payable, net	1,109.4		1,253.7
Debt	578.7		578.7
Current income taxes payable, net	27.1		0.8	[1]
Accrued underwriting expenses and other liabilities	154.8		226.0	[1]
Operating lease liabilities	99.2		105.7	[1]
Due to (from) affiliates	0.0		0.0
Total liabilities	8,666.7		8,733.4	[1]
Total shareholders' equity	1,730.0	$ 1,636.8	1,781.1	[1]	$ 1,929.0	$ 1,880.6	$ 1,746.7
Total liabilities and shareholders' equity	10,396.7		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
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	(1,904.2)		(1,916.7)
Total assets	(1,904.2)		(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		0.0
Accrued underwriting expenses and other liabilities	0.0		0.0
Operating lease liabilities	0.0		0.0
Due to (from) affiliates	(67.3)		(19.3)
Total liabilities	(67.3)		(19.3)
Total shareholders' equity	(1,836.9)		(1,897.4)
Total liabilities and shareholders' equity	(1,904.2)		(1,916.7)
Other Subsidiaries and Eliminations
Assets
Investments	1,448.9		1,693.2
Cash	144.2		104.3
Accrued investment income	6.0		7.5
Premiums receivable	501.8		456.9
Reinsurance recoverables	1,202.5		1,415.2
Goodwill and other intangible assets, net	89.2		89.2
Deferred tax asset, net	7.2		5.7
Deferred acquisition costs, net	67.5		71.8
Ceded unearned premiums	319.1		238.6
Operating lease right-of-use assets	25.1		25.1
Other assets	258.9		213.5
Assets held for sale	0.3		0.0
Intercompany note receivable	(72.9)		(56.7)
Investments in subsidiaries	0.0		0.0
Total assets	3,997.8		4,264.3
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,034.6		2,120.1
Unearned premiums	580.5		511.1
Funds held and ceded reinsurance payable, net	542.7		607.8
Debt	140.9		141.0
Current income taxes payable, net	(6.1)		(7.4)
Accrued underwriting expenses and other liabilities	66.0		124.8
Operating lease liabilities	28.2		29.5
Due to (from) affiliates	11.7		13.4
Total liabilities	3,398.5		3,540.3
Total shareholders' equity	599.3		724.0
Total liabilities and shareholders' equity	3,997.8		4,264.3
Argo Group International Holdings, Ltd (Parent Guarantor)
Assets
Investments	0.6		0.6
Cash	1.9		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	39.9		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	6.8		7.1
Other assets	9.6		7.8
Assets held for sale	0.0		0.0
Intercompany note receivable	0.0		0.0
Investments in subsidiaries	1,904.2		1,916.7
Total assets	1,963.0		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	5.4		13.6
Operating lease liabilities	6.9		7.3
Due to (from) affiliates	67.3		19.3
Total liabilities	233.0		193.6
Total shareholders' equity	1,730.0		1,781.1
Total liabilities and shareholders' equity	1,963.0		1,974.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Assets
Investments	3,402.0		3,405.6
Cash	41.6		31.6
Accrued investment income	17.4		18.2
Premiums receivable	298.4		231.3
Reinsurance recoverables	1,740.3		1,689.4
Goodwill and other intangible assets, net	118.5		123.4
Deferred tax asset, net	11.7		0.4
Deferred acquisition costs, net	86.0		88.4
Ceded unearned premiums	337.4		306.4
Operating lease right-of-use assets	55.3		59.6
Other assets	146.2		165.8
Assets held for sale	12.4		15.4
Intercompany note receivable	72.9		56.7
Investments in subsidiaries	0.0		0.0
Total assets	6,340.1		6,192.2
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	3,181.7		3,037.5
Unearned premiums	900.7		899.8
Funds held and ceded reinsurance payable, net	566.7		645.9
Debt	284.4		284.3
Current income taxes payable, net	33.2		8.2
Accrued underwriting expenses and other liabilities	83.4		87.6
Operating lease liabilities	64.1		68.9
Due to (from) affiliates	(11.7)		(13.4)
Total liabilities	5,102.5		5,018.8
Total shareholders' equity	1,237.6		1,173.4
Total liabilities and shareholders' equity	$ 6,340.1		$ 6,192.2

[1] Derived from audited consolidated financial statements.