Schedule II Condensed Financial Information of Registrant - Schedule of Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Cash	$ 146.1	$ 148.8
Operating leases right-of-use assets	81.4	82.0
Other assets	211.6	294.7
Total assets	10,317.8	10,465.8
Liabilities and Shareholders' Equity
Junior subordinated debentures	258.2	257.8
Accrued underwriting expenses and other liabilities	166.6	167.6
Operating lease liabilities	97.7	95.8
Total liabilities	8,582.6	8,608.0
Shareholders' equity	1,735.2	1,857.8	$ 1,763.7	$ 1,735.0
Total liabilities and shareholders' equity	10,317.8	10,465.8
Argo Group International Holdings, Ltd (Parent  Guarantor)
Assets
Short-term investments	14.5	0.6
Investment in subsidiaries	1,740.8	1,881.9
Cash	2.0	3.0	$ 1.9	$ 1.7
Operating leases right-of-use assets	5.3	6.2
Other assets	5.2	14.5
Total assets	1,767.8	1,906.2
Liabilities and Shareholders' Equity
Junior subordinated debentures	28.4	28.4
Accrued underwriting expenses and other liabilities	3.6	6.3
Operating lease liabilities	5.3	6.2
Due to subsidiaries	(4.7)	7.5
Total liabilities	32.6	48.4
Shareholders' equity	1,735.2	1,857.8
Total liabilities and shareholders' equity	$ 1,767.8	$ 1,906.2