Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018		Jun. 30, 2018	Dec. 31, 2017
Assets
Investments	$ 4,877.6	$ 4,787.0	[1]
Cash	182.3	139.2	[1]
Accrued investment income	26.7	27.2	[1]
Premiums receivable	728.0	649.9	[1]
Reinsurance recoverables	2,769.5	2,688.3	[1]
Goodwill and other intangible assets, net	269.6	270.5
Current income taxes receivable, net	3.8	8.2	[1]
Deferred acquisition costs, net	163.9	167.3	[1]
Ceded unearned premiums	593.9	457.7	[1]
Operating lease right-of-use assets	112.6
Other assets	438.8	362.9	[1]
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	0.0	0.0
Total assets	10,166.7	9,558.2	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,735.7	4,654.6	[1]	$ 4,242.9
Unearned premiums	1,404.0	1,300.9	[1]
Funds held and ceded reinsurance payable, net	1,088.0	1,007.7
Debt	579.4	580.2
Deferred tax liabilities, net	25.4	6.2	[1]
Accrued underwriting expenses and other liabilities	279.4	261.9	[1]
Operating lease liabilities	125.8	0.0	[1]
Due to (from) affiliates	0.0	0.0
Intercompany note payable		0.0
Total liabilities	8,237.7	7,811.5	[1]
Total shareholders' equity	1,929.0	1,746.7	[1]	$ 1,797.1	$ 1,819.7
Total liabilities and shareholders' equity	10,166.7	9,558.2	[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
Current income taxes receivable, 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
Other assets	0.0	0.0
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	(2,076.9)	(1,852.7)
Total assets	(2,076.9)	(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
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	0.0	0.0
Operating lease liabilities	0.0
Due to (from) affiliates	(32.6)	(8.5)
Intercompany note payable		0.0
Total liabilities	(32.6)	(8.5)
Total shareholders' equity	(2,044.3)	(1,844.2)
Total liabilities and shareholders' equity	(2,076.9)	(1,852.7)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,582.4	1,607.3
Cash	152.1	105.8
Accrued investment income	7.3	6.9
Premiums receivable	476.9	420.4
Reinsurance recoverables	1,114.8	1,053.1
Goodwill and other intangible assets, net	104.8	104.8
Current income taxes receivable, net	(0.3)	(0.9)
Deferred acquisition costs, net	78.8	81.1
Ceded unearned premiums	311.2	207.3
Operating lease right-of-use assets	30.2
Other assets	257.5	181.9
Intercompany note receivable	(55.2)	(53.7)
Investments in subsidiaries	0.0	0.0
Total assets	4,060.5	3,714.0
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,865.9	1,883.2
Unearned premiums	589.1	503.5
Funds held and ceded reinsurance payable, net	392.0	268.4
Debt	141.2	142.1
Deferred tax liabilities, net	(3.2)	0.6
Accrued underwriting expenses and other liabilities	170.5	142.3
Operating lease liabilities	33.2
Due to (from) affiliates	14.2	(2.0)
Intercompany note payable		(19.1)
Total liabilities	3,202.9	2,919.0
Total shareholders' equity	857.6	795.0
Total liabilities and shareholders' equity	4,060.5	3,714.0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	1.0	3.8
Cash	1.9	1.7
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	41.2	41.9
Current income taxes receivable, 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	8.2
Other assets	9.9	15.7
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	2,076.9	1,852.7
Total assets	2,139.1	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
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	15.2	7.2
Operating lease liabilities	8.9
Due to (from) affiliates	32.6	8.5
Intercompany note payable		0.0
Total liabilities	210.1	169.1
Total shareholders' equity	1,929.0	1,746.7
Total liabilities and shareholders' equity	2,139.1	1,915.8
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,294.2	3,175.9
Cash	28.3	31.7
Accrued investment income	19.4	20.3
Premiums receivable	251.1	229.5
Reinsurance recoverables	1,654.7	1,635.2
Goodwill and other intangible assets, net	123.6	123.8
Current income taxes receivable, net	4.1	9.1
Deferred acquisition costs, net	85.1	86.2
Ceded unearned premiums	282.7	250.4
Operating lease right-of-use assets	74.2
Other assets	171.4	165.3
Intercompany note receivable	55.2	53.7
Investments in subsidiaries	0.0	0.0
Total assets	6,044.0	5,781.1
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,869.8	2,771.4
Unearned premiums	814.9	797.4
Funds held and ceded reinsurance payable, net	696.0	739.3
Debt	284.8	284.7
Deferred tax liabilities, net	28.6	5.6
Accrued underwriting expenses and other liabilities	93.7	112.4
Operating lease liabilities	83.7
Due to (from) affiliates	(14.2)	2.0
Intercompany note payable		19.1
Total liabilities	4,857.3	4,731.9
Total shareholders' equity	1,186.7	1,049.2
Total liabilities and shareholders' equity	$ 6,044.0	$ 5,781.1

[1] Derived from audited consolidated financial statements.