Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017		Sep. 30, 2017	Dec. 31, 2016
Assets
Investments	$ 4,864.7	$ 4,742.9	[1]
Cash	118.5	176.6	[1]	$ 235.8	$ 86.0
Accrued investment income	26.3	23.5	[1]
Premiums receivable	708.9	598.6	[1]
Reinsurance recoverables	2,208.2	2,093.3	[1]
Goodwill and other intangible assets, net	271.9	258.2
Current income taxes receivable, net	0.0	1.4	[1]
Deferred acquisition costs, net	174.6	160.4	[1]
Ceded unearned premiums	515.6	399.5	[1]
Other assets	391.0	309.6	[1]
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	0.0	0.0
Total assets	9,279.7	8,764.0	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,291.7	4,201.0	[1]	4,305.9
Unearned premiums	1,387.5	1,207.7	[1]
Funds held and ceded reinsurance payable, net	924.2	776.7
Debt	580.6	580.7
Current income taxes payable, net	8.1	0.0	[1]
Deferred tax liabilities, net	14.0	31.3	[1]
Accrued underwriting expenses and other liabilities	255.5	146.9
Due to (from) affiliates	0.0	0.0
Intercompany note payable	0.0
Total liabilities	7,461.6	6,944.3	[1]
Total shareholders' equity	1,818.1	1,819.7	[1]	1,808.0	1,792.7
Total liabilities and shareholders' equity	9,279.7	8,764.0	[1]
Argo Group [Member]
Assets
Investments	0.4	0.4
Cash	1.7	0.9		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	42.2	43.2
Current income taxes receivable, net		0.0
Deferred acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Other assets	8.4	9.2
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	1,941.9	1,940.0
Total assets	1,994.6	1,993.7
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
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	13.1	8.9
Due to (from) affiliates	10.0	11.7
Intercompany note payable	0.0
Total liabilities	176.5	174.0
Total shareholders' equity	1,818.1	1,819.7
Total liabilities and shareholders' equity	1,994.6	1,993.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,205.9	3,079.0
Cash	34.0	47.8		73.5	53.7
Accrued investment income	18.6	18.0
Premiums receivable	239.6	216.5
Reinsurance recoverables	1,559.7	1,487.3
Goodwill and other intangible assets, net	124.0	124.9
Current income taxes receivable, net		2.4
Deferred acquisition costs, net	89.9	80.7
Ceded unearned premiums	236.0	198.5
Other assets	193.7	171.5
Intercompany note receivable	53.0	50.9
Investments in subsidiaries	0.0	0.0
Total assets	5,754.4	5,477.5
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,645.3	2,483.9
Unearned premiums	794.2	704.0
Funds held and ceded reinsurance payable, net	745.2	799.4
Debt	284.7	284.5
Current income taxes payable, net	8.9
Deferred tax liabilities, net	15.5	32.5
Accrued underwriting expenses and other liabilities	94.6	95.0
Due to (from) affiliates	5.3	(0.4)
Intercompany note payable	18.9
Total liabilities	4,612.6	4,398.9
Total shareholders' equity	1,141.8	1,078.6
Total liabilities and shareholders' equity	5,754.4	5,477.5
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,658.4	1,663.5
Cash	82.8	127.9		162.3	32.3
Accrued investment income	7.7	5.5
Premiums receivable	469.3	382.1
Reinsurance recoverables	648.5	606.0
Goodwill and other intangible assets, net	105.7	90.1
Current income taxes receivable, net		(1.0)
Deferred acquisition costs, net	84.7	79.7
Ceded unearned premiums	279.6	201.0
Other assets	188.9	128.9
Intercompany note receivable	(53.0)	(50.9)
Investments in subsidiaries	0.0	0.0
Total assets	3,472.6	3,232.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,646.4	1,717.1
Unearned premiums	593.3	503.7
Funds held and ceded reinsurance payable, net	179.0	(22.7)
Debt	142.5	142.8
Current income taxes payable, net	(0.8)
Deferred tax liabilities, net	(1.5)	(1.2)
Accrued underwriting expenses and other liabilities	147.8	43.0
Due to (from) affiliates	(5.3)	0.4
Intercompany note payable	(18.9)
Total liabilities	2,682.5	2,383.1
Total shareholders' equity	790.1	849.7
Total liabilities and shareholders' equity	3,472.6	3,232.8
Consolidating Adjustments [Member]
Assets
Investments	0.0	0.0
Cash	0.0	0.0		$ 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 acquisition costs, net	0.0	0.0
Ceded unearned premiums	0.0	0.0
Other assets	0.0	0.0
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	(1,941.9)	(1,940.0)
Total assets	(1,941.9)	(1,940.0)
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
Deferred tax liabilities, net	0.0	0.0
Accrued underwriting expenses and other liabilities	0.0	0.0
Due to (from) affiliates	(10.0)	(11.7)
Intercompany note payable	0.0
Total liabilities	(10.0)	(11.7)
Total shareholders' equity	(1,931.9)	(1,928.3)
Total liabilities and shareholders' equity	$ (1,941.9)	$ (1,940.0)

[1] Derived from audited consolidated financial statements.