Senior Unsecured Fixed Rate Notes - Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017		Jun. 30, 2017	Dec. 31, 2016
Assets
Investments	$ 4,768.9	$ 4,742.9	[1]
Cash	126.7	176.6	[1]	$ 214.5	$ 86.0
Accrued investment income	24.9	23.5	[1]
Premiums receivable	698.1	598.6	[1]
Reinsurance recoverables	2,131.6	2,093.3	[1]
Goodwill and other intangible assets, net	273.3	258.2
Current income taxes receivable, net	0.0	1.4	[1]
Deferred acquisition costs, net	161.1	160.4	[1]
Ceded unearned premiums	493.4	399.5	[1]
Other assets	427.7	309.6	[1]
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	0.0	0.0
Total assets	9,105.7	8,764.0	[1]
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	4,242.9	4,201.0	[1]	3,632.8
Unearned premiums	1,283.6	1,207.7	[1]
Funds held and ceded reinsurance payable, net	856.8	776.7
Debt	581.1	580.7
Current income taxes payable, net	13.5	0.0	[1]
Deferred tax liabilities, net	15.1	31.3	[1]
Accrued underwriting expenses and other liabilities	315.6	146.9
Due to (from) affiliates	0.0	0.0
Total liabilities	7,308.6	6,944.3	[1]
Total shareholders' equity	1,797.1	1,819.7	[1]	1,891.3	1,792.7
Total liabilities and shareholders' equity	9,105.7	8,764.0	[1]
Argo Group [Member]
Assets
Investments	1.2	0.4
Cash	1.0	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	42.6	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	9.6	9.2
Intercompany note receivable	0.0	0.0
Investments in subsidiaries	1,968.3	1,940.0
Total assets	2,022.7	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	11.2	8.9
Due to (from) affiliates	61.0	11.7
Total liabilities	225.6	174.0
Total shareholders' equity	1,797.1	1,819.7
Total liabilities and shareholders' equity	2,022.7	1,993.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Assets
Investments	3,107.6	3,079.0
Cash	19.3	47.8		46.2	53.7
Accrued investment income	18.6	18.0
Premiums receivable	221.8	216.5
Reinsurance recoverables	1,548.5	1,487.3
Goodwill and other intangible assets, net	124.6	124.9
Current income taxes receivable, net		2.4
Deferred acquisition costs, net	82.9	80.7
Ceded unearned premiums	221.9	198.5
Other assets	178.4	171.5
Intercompany note receivable	52.2	50.9
Investments in subsidiaries	0.0	0.0
Total assets	5,575.8	5,477.5
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	2,575.3	2,483.9
Unearned premiums	721.8	704.0
Funds held and ceded reinsurance payable, net	754.2	799.4
Debt	284.7	284.5
Current income taxes payable, net	14.4
Deferred tax liabilities, net	15.4	32.5
Accrued underwriting expenses and other liabilities	94.6	95.0
Due to (from) affiliates	1.0	(0.4)
Total liabilities	4,461.4	4,398.9
Total shareholders' equity	1,114.4	1,078.6
Total liabilities and shareholders' equity	5,575.8	5,477.5
Other Subsidiaries and Eliminations [Member]
Assets
Investments	1,660.1	1,663.5
Cash	106.4	127.9		168.3	32.3
Accrued investment income	6.3	5.5
Premiums receivable	476.3	382.1
Reinsurance recoverables	583.1	606.0
Goodwill and other intangible assets, net	106.1	90.1
Current income taxes receivable, net		(1.0)
Deferred acquisition costs, net	78.2	79.7
Ceded unearned premiums	271.5	201.0
Other assets	239.7	128.9
Intercompany note receivable	(52.2)	(50.9)
Investments in subsidiaries	0.0	0.0
Total assets	3,475.5	3,232.8
Liabilities and Shareholders' Equity
Reserves for losses and loss adjustment expenses	1,667.6	1,717.1
Unearned premiums	561.8	503.7
Funds held and ceded reinsurance payable, net	102.6	(22.7)
Debt	143.0	142.8
Current income taxes payable, net	(0.9)
Deferred tax liabilities, net	(0.3)	(1.2)
Accrued underwriting expenses and other liabilities	209.8	43.0
Due to (from) affiliates	(1.0)	0.4
Total liabilities	2,682.6	2,383.1
Total shareholders' equity	792.9	849.7
Total liabilities and shareholders' equity	3,475.5	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	(1,968.3)	(1,940.0)
Total assets	(1,968.3)	(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	(61.0)	(11.7)
Total liabilities	(61.0)	(11.7)
Total shareholders' equity	(1,907.3)	(1,928.3)
Total liabilities and shareholders' equity	$ (1,968.3)	$ (1,940.0)

[1] Derived from audited consolidated financial statements.