Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Premiums and other revenue:
Earned premiums	$ 446.9	$ 389.3	$ 1,279.3	$ 1,167.8
Net investment income	34.5	30.9	103.7	105.0
Fee and other income	3.3	13.0	7.2	20.4
Net realized investment gains before income taxes	14.2	6.0	9.0	25.1
Total revenue	498.9	439.2	1,399.2	1,318.3
Expenses:
Losses and loss adjustment expenses	277.5	326.4	760.2	779.5
Underwriting, acquisition and insurance expenses	168.0	166.1	485.0	474.4
Interest expense	7.9	7.5	23.4	20.4
Fee and other expense	1.9	5.0	5.5	12.4
Foreign currency exchange (gains) losses	(1.7)	0.1	(2.3)	4.0
Total expenses	453.6	505.1	1,271.8	1,290.7
Income (loss) before income taxes	45.3	(65.9)	127.4	27.6
Income tax provision (benefit)	4.7	(4.6)	20.2	6.2
Net (loss) income before equity in earnings of subsidiaries	40.6	(61.3)	107.2	21.4
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	40.6	(61.3)	107.2	21.4
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 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 (loss) before income taxes	0.0	0.0	0.0	0.0
Income tax provision (benefit)	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	(46.4)	53.0	(125.4)	(44.9)
Net income (loss)	(46.4)	53.0	(125.4)	(44.9)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	224.0	244.0	632.5	756.1
Net investment income	14.0	15.5	43.5	43.5
Fee and other income	1.1	0.7	3.5	2.4
Net realized investment gains before income taxes	(1.7)	(0.6)	(6.1)	(1.0)
Total revenue	237.4	259.6	673.4	801.0
Expenses:
Losses and loss adjustment expenses	140.6	227.3	413.5	531.8
Underwriting, acquisition and insurance expenses	79.7	86.5	223.3	265.9
Interest expense	1.7	1.7	5.5	4.7
Fee and other expense	0.7	0.9	1.8	1.7
Foreign currency exchange (gains) losses	(1.7)	0.2	(2.5)	4.1
Total expenses	221.0	316.6	641.6	808.2
Income (loss) before income taxes	16.4	(57.0)	31.8	(7.2)
Income tax provision (benefit)	0.0	(4.0)	2.3	(7.7)
Net (loss) income before equity in earnings of subsidiaries	16.4	(53.0)	29.5	0.5
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	16.4	(53.0)	29.5	0.5
Argo Group [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment income	(0.6)	(1.1)	(2.0)	(3.2)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment gains before income taxes	(0.3)	0.0	(0.2)	0.5
Total revenue	(0.9)	(1.1)	(2.2)	(2.7)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	3.3	5.6	11.5	17.8
Interest expense	1.6	1.6	4.5	3.0
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	4.9	7.2	16.0	20.8
Income (loss) before income taxes	(5.8)	(8.3)	(18.2)	(23.5)
Income tax provision (benefit)	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(5.8)	(8.3)	(18.2)	(23.5)
Equity in undistributed earnings of subsidiaries	46.4	(53.0)	125.4	44.9
Net income (loss)	40.6	(61.3)	107.2	21.4
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	222.9	145.3	646.8	411.7
Net investment income	21.1	16.5	62.2	64.7
Fee and other income	2.2	12.3	3.7	18.0
Net realized investment gains before income taxes	16.2	6.6	15.3	25.6
Total revenue	262.4	180.7	728.0	520.0
Expenses:
Losses and loss adjustment expenses	136.9	99.1	346.7	247.7
Underwriting, acquisition and insurance expenses	85.0	74.0	250.2	190.7
Interest expense	4.6	4.2	13.4	12.7
Fee and other expense	1.2	4.1	3.7	10.7
Foreign currency exchange (gains) losses	0.0	(0.1)	0.2	(0.1)
Total expenses	227.7	181.3	614.2	461.7
Income (loss) before income taxes	34.7	(0.6)	113.8	58.3
Income tax provision (benefit)	4.7	(0.6)	17.9	13.9
Net (loss) income before equity in earnings of subsidiaries	30.0	0.0	95.9	44.4
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income (loss)	$ 30.0	$ 0.0	$ 95.9	$ 44.4