Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Income (Detail) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
Net investment expense		$ 28.6	$ 121.3	$ 129.8	$ 187.6
Net investment and other (gains) losses		(0.3)	(22.7)	(115.3)	32.6
Total revenue		190.6	1,324.5	1,754.9	2,130.3
Expenses:
Interest expense		3.5	29.8	26.8	21.6
Non-operating expenses		13.1	41.1	51.5	43.7
Total expenses		188.4	1,534.6	1,938.1	2,125.0
Net income (loss)	$ 0.9	0.9	(210.4)	(175.2)	6.7
Dividends on Series A Preferred stock		0.0	10.5	10.5	10.5
Net income (loss) attributable to common shareholders, basic		0.9	(220.9)	(185.7)	(3.8)
Argo Group International Holdings, Ltd (Parent  Guarantor)
Revenue:
Net investment expense		(0.1)	0.0	0.0	0.0
Total revenue		(0.1)	0.0	0.0	0.0
Expenses:
Interest expense		0.4	2.3	1.6	1.2
Operating expenses		(1.3)	6.8	(0.6)	6.2
Non-operating expenses		2.7	21.3	26.2	0.2
Total expenses		1.8	30.4	27.2	7.6
Net income before equity in earnings of subsidiaries		(1.9)	(30.4)	(27.2)	(7.6)
Equity in undistributed earnings of subsidiaries		2.8	(180.0)	(148.0)	14.3
Net income (loss)		0.9	(210.4)	(175.2)	6.7
Dividends on Series A Preferred stock		0.0	10.5	10.5	10.5
Net income (loss) attributable to common shareholders, basic		$ 0.9	$ (220.9)	$ (185.7)	$ (3.8)