Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue:
Net investment expense	$ 187.6	$ 112.7	$ 151.1
Net realized investment (gains) losses	32.6	(3.6)	80.1
Total revenue	2,130.3	1,889.6	1,960.9
Expenses:
Interest expense	21.6	26.9	34.1
Non-operating expenses	43.7	21.1	37.6
Total expenses	2,125.0	1,936.0	1,960.9
Net income (loss)	6.7	(54.1)	(14.1)
Dividends on preferred shares	10.5	4.6	0.0
Net income (loss) attributable to common shareholders, basic	(3.8)	(58.7)	(14.1)
Argo Group International Holdings, Ltd (Parent  Guarantor)
Revenue:
Net investment expense	0.0	0.0	(2.9)
Net realized investment (gains) losses	0.0	(8.3)	(0.1)
Total revenue	0.0	(8.3)	(3.0)
Expenses:
Interest expense	1.2	3.6	6.6
Operating expenses	6.2	20.4	1.3
Non-operating expenses	0.2	4.4	26.8
Total expenses	7.6	28.4	34.7
Net income before equity in earnings of subsidiaries	(7.6)	(36.7)	(37.7)
Equity in undistributed earnings of subsidiaries	14.3	(17.4)	23.6
Net income (loss)	6.7	(54.1)	(14.1)
Dividends on preferred shares	10.5	4.6	0.0
Net income (loss) attributable to common shareholders, basic	$ (3.8)	$ (58.7)	$ (14.1)