Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Premiums and other revenue:
Earned premiums	$ 431.7	$ 417.7	$ 852.2	$ 832.4	$ 832.4
Net investment (expense) income	42.8	33.2	76.7	69.2	69.2
Fee and other income	2.1	1.9	4.4	3.9	3.9
Net realized investment (losses) gains	12.0	10.5	64.5	(5.2)	(5.2)
Total revenue	488.6	463.3	997.8	900.3	900.3
Expenses:
Losses and loss adjustment expenses	284.8	245.5	522.7	482.7
Underwriting, acquisition and insurance expenses	168.9	156.8	329.6	317.0	317.0
Interest expense	9.3	7.8	17.8	15.5
Fee and other expense	1.3	1.6	2.6	3.6
Foreign currency exchange gains	(5.3)	(5.5)	(4.6)	(0.6)	(0.6)
Total expenses	459.0	406.2	868.1	818.2
Income before income taxes	29.6	57.1	129.7	82.1	82.1
Income tax provision	0.8	15.3	9.7	15.5	15.5
Net (loss) income before equity in earnings of subsidiaries	28.8	41.8	120.0	66.6
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	28.8	41.8	120.0	66.6	$ 66.6
Consolidating Adjustments [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	0.0	0.0	0.0	0.0
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment (losses) gains	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	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	(39.4)	(49.4)	(133.8)	(79.0)
Net income	(39.4)	(49.4)	(133.8)	(79.0)
Other Subsidiaries and Eliminations [Member]
Premiums and other revenue:
Earned premiums	171.6	150.7	325.6	408.5
Net investment (expense) income	15.3	14.9	26.1	29.5
Fee and other income	1.4	1.2	2.6	2.4
Net realized investment (losses) gains	(2.9)	(4.5)	3.8	(4.4)
Total revenue	185.4	162.3	358.1	436.0
Expenses:
Losses and loss adjustment expenses	113.5	131.1	193.4	272.9
Underwriting, acquisition and insurance expenses	56.5	55.5	112.4	143.6
Interest expense	2.4	1.7	4.5	3.8
Fee and other expense	0.4	0.5	0.9	1.1
Foreign currency exchange gains	(5.2)	(5.6)	(4.8)	(0.8)
Total expenses	167.6	183.2	306.4	420.6
Income before income taxes	17.8	(20.9)	51.7	15.4
Income tax provision	(2.5)	0.7	(3.6)	2.3
Net (loss) income before equity in earnings of subsidiaries	20.3	(21.6)	55.3	13.1
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	20.3	(21.6)	55.3	13.1
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	(0.6)	(0.7)	(1.3)	(1.4)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment (losses) gains	(0.1)	0.5	(0.2)	0.1
Total revenue	(0.7)	(0.2)	(1.5)	(1.3)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	8.2	5.8	8.9	8.2
Interest expense	1.7	1.6	3.4	2.9
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange gains	0.0	0.0	0.0	0.0
Total expenses	9.9	7.4	12.3	11.1
Income before income taxes	(10.6)	(7.6)	(13.8)	(12.4)
Income tax provision	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(10.6)	(7.6)	(13.8)	(12.4)
Equity in undistributed earnings of subsidiaries	39.4	49.4	133.8	79.0
Net income	28.8	41.8	120.0	66.6
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Premiums and other revenue:
Earned premiums	260.1	267.0	526.6	423.9
Net investment (expense) income	28.1	19.0	51.9	41.1
Fee and other income	0.7	0.7	1.8	1.5
Net realized investment (losses) gains	15.0	14.5	60.9	(0.9)
Total revenue	303.9	301.2	641.2	465.6
Expenses:
Losses and loss adjustment expenses	171.3	114.4	329.3	209.8
Underwriting, acquisition and insurance expenses	104.2	95.5	208.3	165.2
Interest expense	5.2	4.5	9.9	8.8
Fee and other expense	0.9	1.1	1.7	2.5
Foreign currency exchange gains	(0.1)	0.1	0.2	0.2
Total expenses	281.5	215.6	549.4	386.5
Income before income taxes	22.4	85.6	91.8	79.1
Income tax provision	3.3	14.6	13.3	13.2
Net (loss) income before equity in earnings of subsidiaries	19.1	71.0	78.5	65.9
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net income	$ 19.1	$ 71.0	$ 78.5	$ 65.9