Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Premiums and other revenue:
Earned premiums	$ 417.7	$ 399.1	$ 832.4	$ 778.5
Net investment income	33.2	43.6	69.2	74.1
Fee and other income	1.9	3.8	3.9	7.4
Net realized investment gains (losses) before income taxes	10.5	4.5	(5.2)	19.1
Total revenue	463.3	451.0	900.3	879.1
Expenses:
Losses and loss adjustment expenses	245.5	230.6	482.7	453.1
Underwriting, acquisition and insurance expenses	156.8	154.7	317.0	308.3
Interest expense	7.8	7.0	15.5	12.9
Fee and other expense	1.6	3.3	3.6	7.4
Foreign currency exchange (gains) losses	(5.5)	4.6	(0.6)	3.9
Total expenses	406.2	400.2	818.2	785.6
Income before income taxes	57.1	50.8	82.1	93.5
Income tax provision	15.3	4.8	15.5	10.8
Net (loss) income before equity in earnings of subsidiaries	41.8	46.0	66.6	82.7
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	41.8	46.0	66.6	82.7
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment gains (losses) before income taxes	0.0	0.0	0.0	0.0
Total revenue	0.0	0.0	0.0	0.0
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange (gains) losses	0.0	0.0	0.0	0.0
Total expenses	0.0	0.0	0.0	0.0
Income before income taxes	0.0	0.0	0.0	0.0
Income tax provision	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Equity in undistributed earnings of subsidiaries	(49.4)	(51.3)	(79.0)	(97.9)
Net income	(49.4)	(51.3)	(79.0)	(97.9)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	150.7	263.6	408.5	512.1
Net investment income	14.9	14.7	29.5	28.0
Fee and other income	1.2	0.9	2.4	1.7
Net realized investment gains (losses) before income taxes	(4.5)	(0.7)	(4.4)	(0.4)
Total revenue	162.3	278.5	436.0	541.4
Expenses:
Losses and loss adjustment expenses	131.1	158.1	272.9	304.5
Underwriting, acquisition and insurance expenses	55.5	92.4	143.6	179.4
Interest expense	1.7	1.7	3.8	3.0
Fee and other expense	0.5	0.1	1.1	0.8
Foreign currency exchange (gains) losses	(5.6)	4.7	(0.8)	3.9
Total expenses	183.2	257.0	420.6	491.6
Income before income taxes	(20.9)	21.5	15.4	49.8
Income tax provision	0.7	(3.3)	2.3	(3.7)
Net (loss) income before equity in earnings of subsidiaries	(21.6)	24.8	13.1	53.5
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	(21.6)	24.8	13.1	53.5
Argo Group [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment income	(0.7)	(0.4)	(1.4)	(2.1)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment gains (losses) before income taxes	0.5	0.0	0.1	0.5
Total revenue	(0.2)	(0.4)	(1.3)	(1.6)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	5.8	3.8	8.2	12.2
Interest expense	1.6	1.1	2.9	1.4
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange (gains) losses	0.0	0.0	0.0	0.0
Total expenses	7.4	4.9	11.1	13.6
Income before income taxes	(7.6)	(5.3)	(12.4)	(15.2)
Income tax provision	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(7.6)	(5.3)	(12.4)	(15.2)
Equity in undistributed earnings of subsidiaries	49.4	51.3	79.0	97.9
Net income	41.8	46.0	66.6	82.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	267.0	135.5	423.9	266.4
Net investment income	19.0	29.3	41.1	48.2
Fee and other income	0.7	2.9	1.5	5.7
Net realized investment gains (losses) before income taxes	14.5	5.2	(0.9)	19.0
Total revenue	301.2	172.9	465.6	339.3
Expenses:
Losses and loss adjustment expenses	114.4	72.5	209.8	148.6
Underwriting, acquisition and insurance expenses	95.5	58.5	165.2	116.7
Interest expense	4.5	4.2	8.8	8.5
Fee and other expense	1.1	3.2	2.5	6.6
Foreign currency exchange (gains) losses	0.1	(0.1)	0.2	0.0
Total expenses	215.6	138.3	386.5	280.4
Income before income taxes	85.6	34.6	79.1	58.9
Income tax provision	14.6	8.1	13.2	14.5
Net (loss) income before equity in earnings of subsidiaries	71.0	26.5	65.9	44.4
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	$ 71.0	$ 26.5	$ 65.9	$ 44.4