Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums and other revenue:
Earned premiums										$ 1,572.3	$ 1,410.8	$ 1,371.9
Net investment income										140.0	115.1	88.6
Fee and other income										22.5	24.5	22.2
Net realized investment and other gains										39.3	26.1	24.1
Total revenue		$ 455.8	$ 439.2	$ 451.0	$ 428.1	$ 405.4	$ 416.7	$ 384.3	$ 370.1	1,774.1	1,576.5	1,506.8
Expenses:
Losses and loss adjustment expenses										1,050.2	810.1	766.1
Underwriting, acquisition and insurance expenses										635.4	547.0	536.7
Interest expense										27.7	19.6	19.0
Fee and other expense										14.6	22.4	25.8
Foreign currency exchange losses (gains)										6.3	(4.5)	(18.3)
Total expenses										1,734.2	1,394.6	1,329.3
Income before income taxes		12.3	(65.9)	50.8	42.7	47.0	62.2	39.6	33.1	39.9	181.9	177.5
Provision for income taxes										(10.4)	35.2	14.3
Net (loss) income before equity in earnings of subsidiaries										50.3	146.7	163.2
Equity in undistributed earnings of subsidiaries										0.0	0.0	0.0
Net income		$ 28.9	$ (61.3)	$ 46.0	$ 36.7	$ 32.9	$ 55.2	$ 30.9	$ 27.7	50.3	146.7	163.2
Argo Group [Member]
Premiums and other revenue:
Earned premiums										0.0	0.0	0.0
Net investment income										(4.5)	37.2	40.1
Fee and other income										0.0	0.0	0.0
Net realized investment and other gains										0.4	0.6	0.0
Total revenue										(4.1)	37.8	40.1
Expenses:
Losses and loss adjustment expenses										0.0	0.0	0.0
Underwriting, acquisition and insurance expenses										14.3	12.8	19.0
Interest expense										4.3	1.4	1.5
Fee and other expense										0.0	0.0	0.0
Foreign currency exchange losses (gains)										0.1	0.0	0.0
Total expenses										18.7	14.2	20.5
Income before income taxes										(22.8)	23.6	19.6
Provision for income taxes										0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries										(22.8)	23.6	19.6
Equity in undistributed earnings of subsidiaries										73.1	123.1	143.6
Net income										50.3	146.7	163.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums										555.9	497.3	497.3
Net investment income										87.5	78.9	59.0
Fee and other income										18.8	21.4	19.0
Net realized investment and other gains										40.8	50.0	31.8
Total revenue										703.0	647.6	607.1
Expenses:
Losses and loss adjustment expenses										337.9	290.4	304.2
Underwriting, acquisition and insurance expenses										266.6	204.4	198.4
Interest expense										17.2	15.8	15.3
Fee and other expense										12.4	21.7	23.3
Foreign currency exchange losses (gains)										0.1	0.2	1.0
Total expenses										634.2	532.5	542.2
Income before income taxes										68.8	115.1	64.9
Provision for income taxes										(0.6)	33.6	12.9
Net (loss) income before equity in earnings of subsidiaries										69.4	81.5	52.0
Equity in undistributed earnings of subsidiaries										0.0	0.0	0.0
Net income										69.4	81.5	52.0
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	[1]									1,016.4	913.5	874.6
Net investment income	[1]									57.0	40.0	30.5
Fee and other income	[1]									3.7	3.1	3.2
Net realized investment and other gains	[1]									(1.9)	(24.5)	(7.7)
Total revenue	[1]									1,075.2	932.1	900.6
Expenses:
Losses and loss adjustment expenses	[1]									712.3	519.7	461.9
Underwriting, acquisition and insurance expenses	[1]									354.5	329.8	319.3
Interest expense	[1]									6.2	2.4	2.2
Fee and other expense	[1]									2.2	0.7	2.5
Foreign currency exchange losses (gains)	[1]									6.1	(4.7)	(19.3)
Total expenses	[1]									1,081.3	847.9	766.6
Income before income taxes	[1]									(6.1)	84.2	134.0
Provision for income taxes	[1]									(9.8)	1.6	1.4
Net (loss) income before equity in earnings of subsidiaries	[1]									3.7	82.6	132.6
Equity in undistributed earnings of subsidiaries	[1]									0.0	0.0	0.0
Net income	[1]									3.7	82.6	132.6
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	[2]									0.0	0.0	0.0
Net investment income	[2]									0.0	(41.0)	(41.0)
Fee and other income	[2]									0.0	0.0	0.0
Net realized investment and other gains	[2]									0.0	0.0	0.0
Total revenue	[2]									0.0	(41.0)	(41.0)
Expenses:
Losses and loss adjustment expenses	[2]									0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	[2]									0.0	0.0	0.0
Interest expense	[2]									0.0	0.0	0.0
Fee and other expense	[2]									0.0	0.0	0.0
Foreign currency exchange losses (gains)	[2]									0.0	0.0	0.0
Total expenses	[2]									0.0	0.0	0.0
Income before income taxes	[2]									0.0	(41.0)	(41.0)
Provision for income taxes	[2]									0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	[2]									0.0	(41.0)	(41.0)
Equity in undistributed earnings of subsidiaries	[2]									(73.1)	(123.1)	(143.6)
Net income	[2]									$ (73.1)	$ (164.1)	$ (184.6)

[1] Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.

[2] Includes all Argo Group parent company eliminations.