Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement 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
Premiums and other revenue:
Earned premiums	$ 425,800,000	$ 451,500,000	$ 431,700,000	$ 420,500,000	$ 452,400,000	$ 446,900,000	$ 417,700,000	$ 414,700,000	$ 1,729,500,000	$ 1,731,700,000	$ 1,572,300,000
Net investment income									151,100,000	133,100,000	140,000,000.0
Fee and other income									9,100,000	9,000,000.0	22,500,000
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:
Losses and loss adjustment expenses	359,200,000	338,800,000	284,800,000	237,900,000	280,600,000	277,500,000	245,500,000	237,200,000	1,220,700,000	1,040,800,000	1,050,200,000
Underwriting, acquisition and insurance expenses	180,200,000	164,000,000.0	161,400,000	160,200,000	169,700,000	168,000,000.0	156,800,000	160,200,000	665,800,000	654,700,000	635,400,000
Other corporate expenses		3,700,000	7,500,000	500,000					37,600,000	0	0
Interest expense									33,600,000	31,600,000	27,700,000
Fee and other expense									5,800,000	7,100,000	14,600,000
Foreign currency exchange (gains) loss									(9,600,000)	(100,000)	6,300,000
Impairment of goodwill	15,600,000								15,600,000	0	0
Total expenses									1,969,500,000	1,734,100,000	1,734,200,000
Net income (loss) before income taxes	(103,300,000)	(26,200,000)	29,600,000	100,100,000	(59,700,000)	45,300,000	57,100,000	25,000,000.0	200,000	67,700,000	39,900,000
Provision (benefit) for income taxes									8,600,000	4,100,000	(10,400,000)
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
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums									0	0	0
Net investment income									(52,100,000)	(36,500,000)	0
Fee and other income									0	0	0
Net realized investment (losses) gains									0	(2,700,000)	0
Total revenue									(52,100,000)	(39,200,000)	0
Expenses:
Losses and loss adjustment expenses									0	0	0
Underwriting, acquisition and insurance expenses									0	0	0
Other corporate expenses									0
Interest expense									0	0	0
Fee and other expense									0	0	0
Foreign currency exchange (gains) loss									0	0	0
Impairment of goodwill									0
Total expenses									0	0	0
Net income (loss) before income taxes									(52,100,000)	(39,200,000)	0
Provision (benefit) for income taxes									0	0	0
Net income (loss) before equity in earnings of subsidiaries									(52,100,000)	(39,200,000)	0
Equity in undistributed earnings of subsidiaries									22,800,000	(44,800,000)	(73,100,000)
Net (loss) income									(29,300,000)	(84,000,000.0)	(73,100,000)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums									685,600,000	869,800,000	1,016,400,000
Net investment income									50,700,000	56,400,000	57,000,000.0
Fee and other income									5,900,000	4,600,000	3,700,000
Net realized investment (losses) gains									(800,000)	(20,500,000)	(1,900,000)
Total revenue									741,400,000	910,300,000	1,075,200,000
Expenses:
Losses and loss adjustment expenses									523,900,000	517,100,000	712,300,000
Underwriting, acquisition and insurance expenses									249,300,000	309,800,000	354,500,000
Other corporate expenses									0
Interest expense									8,600,000	7,200,000	6,200,000
Fee and other expense									1,600,000	1,800,000	2,200,000
Foreign currency exchange (gains) loss									(10,300,000)	(300,000)	6,100,000
Impairment of goodwill									15,600,000
Total expenses									788,700,000	835,600,000	1,081,300,000
Net income (loss) before income taxes									(47,300,000)	74,700,000	(6,100,000)
Provision (benefit) for income taxes									(8,200,000)	4,900,000	(9,800,000)
Net income (loss) before equity in earnings of subsidiaries									(39,100,000)	69,800,000	3,700,000
Equity in undistributed earnings of subsidiaries									0	0	0
Net (loss) income									(39,100,000)	69,800,000	3,700,000
Argo Group [Member]
Premiums and other revenue:
Earned premiums									0	0	0
Net investment income									49,200,000	33,800,000	(4,500,000)
Fee and other income									0	0	0
Net realized investment (losses) gains									(100,000)	2,500,000	400,000
Total revenue									49,100,000	36,300,000	(4,100,000)
Expenses:
Losses and loss adjustment expenses									0	0	0
Underwriting, acquisition and insurance expenses									1,300,000	11,300,000	14,300,000
Other corporate expenses									26,800,000	0	0
Interest expense									6,600,000	6,200,000	4,300,000
Fee and other expense									0	0	0
Foreign currency exchange (gains) loss									0	0	100,000
Impairment of goodwill									0
Total expenses									34,700,000	17,500,000	18,700,000
Net income (loss) before income taxes									14,400,000	18,800,000	(22,800,000)
Provision (benefit) for income taxes									0	0	0
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
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums									1,043,900,000	861,900,000	555,900,000
Net investment income									103,300,000	79,400,000	87,500,000
Fee and other income									3,200,000	4,400,000	18,800,000
Net realized investment (losses) gains									80,900,000	(51,300,000)	40,800,000
Total revenue									1,231,300,000	894,400,000	703,000,000.0
Expenses:
Losses and loss adjustment expenses									696,800,000	523,700,000	337,900,000
Underwriting, acquisition and insurance expenses									415,200,000	333,600,000	266,600,000
Other corporate expenses									10,800,000
Interest expense									18,400,000	18,200,000	17,200,000
Fee and other expense									4,200,000	5,300,000	12,400,000
Foreign currency exchange (gains) loss									700,000	200,000	100,000
Impairment of goodwill									0
Total expenses									1,146,100,000	881,000,000.0	634,200,000
Net income (loss) before income taxes									85,200,000	13,400,000	68,800,000
Provision (benefit) for income taxes									16,800,000	(800,000)	(600,000)
Net income (loss) before equity in earnings of subsidiaries									68,400,000	14,200,000	69,400,000
Equity in undistributed earnings of subsidiaries									0	0	0
Net (loss) income									$ 68,400,000	$ 14,200,000	$ 69,400,000