Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018		Sep. 30, 2018	Dec. 31, 2017
Assets
Investments	$ 5,127.0	$ 4,787.0	[1]
Cash	145.1	139.2	[1]
Accrued investment income	27.7	27.2	[1]
Premiums receivable	740.3	649.9	[1]
Reinsurance recoverables	2,873.8	2,688.3	[1]
Goodwill and other intangible assets, net	269.2	270.5
Current income taxes receivable, net	0.2	8.2	[1]
Deferred acquisition costs, net	159.5	167.3	[1]
Ceded unearned premiums	603.8	457.7	[1]
Operating lease right-of-use assets	108.6
Other assets	390.6	362.9	[1]
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	0.0	0.0
Total assets	10,445.8	9,558.2	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,842.5	4,654.6	[1]	$ 4,291.7
Unearned premiums	1,495.5	1,300.9	[1]
Funds held and ceded reinsurance payable, net	1,211.8	1,007.7
Debt	578.6	580.2
Deferred tax liabilities, net	19.9	6.2	[1]
Accrued underwriting expenses and other liabilities	283.9	261.9	[1]
Operating lease liabilities	120.2	0.0	[1]
Due to (from) affiliates	0.0	0.0
Intercompany note payable		0.0
Total liabilities	8,552.4	7,811.5	[1]
Total shareholders' equity	1,893.4	1,746.7	[1]	$ 1,818.1	$ 1,819.7
Total liabilities and shareholders' equity	10,445.8	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	(2,053.6)	(1,852.7)
Total assets	(2,053.6)	(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	(50.5)	(8.5)
Intercompany note payable		0.0
Total liabilities	(50.5)	(8.5)
Total shareholders' equity	(2,003.1)	(1,844.2)
Total liabilities and shareholders' equity	(2,053.6)	(1,852.7)
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,710.3	1,607.3
Cash	98.8	105.8
Accrued investment income	7.9	6.9
Premiums receivable	478.7	420.4
Reinsurance recoverables	1,211.9	1,053.1
Goodwill and other intangible assets, net	104.8	104.8
Current income taxes receivable, net	0.2	(0.9)
Deferred acquisition costs, net	66.9	81.1
Ceded unearned premiums	308.9	207.3
Operating lease right-of-use assets	28.9
Other assets	209.9	181.9
Intercompany note receivable	(55.9)	(53.7)
Investments in subsidiaries	0.0	0.0
Total assets	4,171.3	3,714.0
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,897.8	1,883.2
Unearned premiums	598.7	503.5
Funds held and ceded reinsurance payable, net	546.4	268.4
Debt	141.0	142.1
Deferred tax liabilities, net	(5.4)	0.6
Accrued underwriting expenses and other liabilities	147.3	142.3
Operating lease liabilities	30.7
Due to (from) affiliates	4.2	(2.0)
Intercompany note payable		(19.1)
Total liabilities	3,360.7	2,919.0
Total shareholders' equity	810.6	795.0
Total liabilities and shareholders' equity	4,171.3	3,714.0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Investments	0.3	3.8
Cash	3.2	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	40.9	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	7.8
Other assets	10.5	15.7
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	2,053.6	1,852.7
Total assets	2,116.3	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	11.0	7.2
Operating lease liabilities	8.0
Due to (from) affiliates	50.5	8.5
Intercompany note payable		0.0
Total liabilities	222.9	169.1
Total shareholders' equity	1,893.4	1,746.7
Total liabilities and shareholders' equity	2,116.3	1,915.8
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,416.4	3,175.9
Cash	43.1	31.7
Accrued investment income	19.8	20.3
Premiums receivable	261.6	229.5
Reinsurance recoverables	1,661.9	1,635.2
Goodwill and other intangible assets, net	123.5	123.8
Current income taxes receivable, net	0.0	9.1
Deferred acquisition costs, net	92.6	86.2
Ceded unearned premiums	294.9	250.4
Operating lease right-of-use assets	71.9
Other assets	170.2	165.3
Intercompany note receivable	55.9	53.7
Investments in subsidiaries	0.0	0.0
Total assets	6,211.8	5,781.1
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,944.7	2,771.4
Unearned premiums	896.8	797.4
Funds held and ceded reinsurance payable, net	665.4	739.3
Debt	284.2	284.7
Deferred tax liabilities, net	25.3	5.6
Accrued underwriting expenses and other liabilities	125.6	112.4
Operating lease liabilities	81.5
Due to (from) affiliates	(4.2)	2.0
Intercompany note payable		19.1
Total liabilities	5,019.3	4,731.9
Total shareholders' equity	1,192.5	1,049.2
Total liabilities and shareholders' equity	$ 6,211.8	$ 5,781.1

[1] Derived from audited consolidated financial statements.