Schedule II Condensed Financial Information of Registrant - Schedule of Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Short-term investments	$ 845.0	$ 482.3
Investment in subsidiaries	0.0	0.0
Cash	137.8	139.2
Goodwill and other intangible assets, net	253.2	270.5
Operating leases right-of-use assets	91.8
Other assets	387.1	362.9
Total assets	10,514.5	9,558.2
Liabilities and Shareholders' Equity
Junior subordinated debentures	257.4	257.0
Other indebtedness	181.3	183.4
Accrued underwriting expenses and other liabilities	226.0	261.9
Operating lease liabilities	105.7
Total liabilities	8,733.4	7,811.5
Total shareholders' equity	1,781.1	1,746.7	$ 1,819.7	$ 1,792.7
Total liabilities and shareholders' equity	10,514.5	9,558.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Assets
Short-term investments	0.6	3.8
Investment in subsidiaries	1,916.7	1,852.7
Cash	1.9	1.7	$ 0.9	$ 0.0
Goodwill and other intangible assets, net	40.6	41.9
Operating leases right-of-use assets	7.1
Other assets	7.8	15.7
Total assets	1,974.7	1,915.8
Liabilities and Shareholders' Equity
Junior subordinated debentures	28.4	28.4
Other indebtedness	125.0	125.0
Accrued underwriting expenses and other liabilities	13.6	7.2
Operating lease liabilities	7.3
Due to subsidiaries	19.3	8.5
Total liabilities	193.6	169.1
Total shareholders' equity	1,781.1	1,746.7
Total liabilities and shareholders' equity	$ 1,974.7	$ 1,915.8