Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Premiums and other revenue:
Earned premiums	$ 433.4	$ 431.7	$ 868.4	$ 852.2
Net investment (expense) income	1.5	42.8	37.0	76.7
Fee and other income	2.0	2.1	4.1	4.4
Net realized investment and other gains (losses) before income taxes	20.1	12.0	(16.0)	64.5
Total revenue	457.0	488.6	893.5	997.8
Expenses:
Losses and loss adjustment expenses	273.2	284.8	554.1	522.7
Underwriting, acquisition and insurance expenses	161.4	161.4	329.4	321.6
Other corporate expenses	2.5	7.5	5.8	8.0
Interest expense	6.8	9.3	14.5	17.8
Fee and other expense	1.0	1.3	2.2	2.6
Foreign currency exchange losses (gains)	6.4	(5.3)	3.4	(4.6)
Total expenses	451.3	459.0	909.4	868.1
Income (loss) before income taxes	5.7	29.6	(15.9)	129.7
Income tax provision	12.1	0.8	9.3	9.7
Net (loss) income before equity in earnings of subsidiaries	(6.4)	28.8	(25.2)	120.0
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(6.4)	28.8	(25.2)	120.0
Consolidating Adjustments
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 and other 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
Other corporate 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 losses (gains)	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	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	0.9	(39.4)	9.8	(133.8)
Net (loss) income	0.9	(39.4)	9.8	(133.8)
Other Subsidiaries and Eliminations
Premiums and other revenue:
Earned premiums	132.7	171.6	266.2	325.6
Net investment (expense) income	(4.3)	15.3	6.6	26.1
Fee and other income	1.4	1.4	2.7	2.6
Net realized investment and other gains (losses) before income taxes	(14.4)	(2.9)	(9.8)	3.8
Total revenue	115.4	185.4	265.7	358.1
Expenses:
Losses and loss adjustment expenses	105.5	113.5	195.9	193.4
Underwriting, acquisition and insurance expenses	56.5	56.5	116.9	112.4
Other corporate expenses	0.0	0.0	0.0	0.0
Interest expense	1.7	2.4	3.7	4.5
Fee and other expense	0.4	0.4	0.9	0.9
Foreign currency exchange losses (gains)	6.4	(5.2)	4.4	(4.8)
Total expenses	170.5	167.6	321.8	306.4
Income (loss) before income taxes	(55.1)	17.8	(56.1)	51.7
Income tax provision	(3.7)	(2.5)	(0.6)	(3.6)
Net (loss) income before equity in earnings of subsidiaries	(51.4)	20.3	(55.5)	55.3
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	(51.4)	20.3	(55.5)	55.3
Argo Group International Holdings, Ltd (Parent Guarantor)
Premiums and other revenue:
Earned premiums	0.0	0.0	0.0	0.0
Net investment (expense) income	0.0	(0.6)	0.0	(1.3)
Fee and other income	0.0	0.0	0.0	0.0
Net realized investment and other gains (losses) before income taxes	1.4	(0.1)	1.3	(0.2)
Total revenue	1.4	(0.7)	1.3	(1.5)
Expenses:
Losses and loss adjustment expenses	0.0	0.0	0.0	0.0
Underwriting, acquisition and insurance expenses	3.2	0.7	9.0	0.9
Other corporate expenses	2.6	7.5	5.2	8.0
Interest expense	1.1	1.7	2.5	3.4
Fee and other expense	0.0	0.0	0.0	0.0
Foreign currency exchange losses (gains)	0.0	0.0	0.0	0.0
Total expenses	6.9	9.9	16.7	12.3
Income (loss) before income taxes	(5.5)	(10.6)	(15.4)	(13.8)
Income tax provision	0.0	0.0	0.0	0.0
Net (loss) income before equity in earnings of subsidiaries	(5.5)	(10.6)	(15.4)	(13.8)
Equity in undistributed earnings of subsidiaries	(0.9)	39.4	(9.8)	133.8
Net (loss) income	(6.4)	28.8	(25.2)	120.0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Premiums and other revenue:
Earned premiums	300.7	260.1	602.2	526.6
Net investment (expense) income	5.8	28.1	30.4	51.9
Fee and other income	0.6	0.7	1.4	1.8
Net realized investment and other gains (losses) before income taxes	33.1	15.0	(7.5)	60.9
Total revenue	340.2	303.9	626.5	641.2
Expenses:
Losses and loss adjustment expenses	167.7	171.3	358.2	329.3
Underwriting, acquisition and insurance expenses	101.7	104.2	203.5	208.3
Other corporate expenses	(0.1)	0.0	0.6	0.0
Interest expense	4.0	5.2	8.3	9.9
Fee and other expense	0.6	0.9	1.3	1.7
Foreign currency exchange losses (gains)	0.0	(0.1)	(1.0)	0.2
Total expenses	273.9	281.5	570.9	549.4
Income (loss) before income taxes	66.3	22.4	55.6	91.8
Income tax provision	15.8	3.3	9.9	13.3
Net (loss) income before equity in earnings of subsidiaries	50.5	19.1	45.7	78.5
Equity in undistributed earnings of subsidiaries	0.0	0.0	0.0	0.0
Net (loss) income	$ 50.5	$ 19.1	$ 45.7	$ 78.5