Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Income (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Net investment (expense) income									$ 151,100,000	$ 133,100,000	$ 140,000,000.0
Net realized investment (losses) gains									80,000,000.0	(72,000,000.0)	39,300,000
Total revenue									1,969,700,000	1,801,800,000	1,774,100,000
Expenses:
Interest expense									33,600,000	31,600,000	27,700,000
Other corporate expenses		$ 3,700,000	$ 7,500,000	$ 500,000					37,600,000	0	0
Foreign currency exchange loss									(9,600,000)	(100,000)	6,300,000
Total expenses									1,969,500,000	1,734,100,000	1,734,200,000
Net income (loss) before equity in earnings of subsidiaries									(8,400,000)	63,600,000	50,300,000
Equity in undistributed earnings of subsidiaries									0	0	0
Net (loss) income	$ (103,300,000)	$ (25,100,000)	$ 28,800,000	$ 91,200,000	$ (43,600,000)	$ 40,600,000	$ 41,800,000	$ 24,800,000	(8,400,000)	63,600,000	50,300,000
Cash dividends										52,100,000		$ 36,500,000
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Revenue:
Net investment (expense) income									49,200,000	33,800,000	(4,500,000)
Net realized investment (losses) gains									(100,000)	2,500,000	400,000
Total revenue									49,100,000	36,300,000	(4,100,000)
Expenses:
Interest expense									6,600,000	6,200,000	4,300,000
Operating expenses									1,300,000	11,300,000	14,300,000
Other corporate expenses									26,800,000	0	0
Foreign currency exchange loss									0	0	100,000
Total expenses									34,700,000	17,500,000	18,700,000
Net income (loss) before equity in earnings of subsidiaries									14,400,000	18,800,000	(22,800,000)
Equity in undistributed earnings of subsidiaries									(22,800,000)	44,800,000	73,100,000
Net (loss) income									$ (8,400,000)	$ 63,600,000	$ 50,300,000