Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net (loss) income	$ (23.1)	$ (5.4)	$ (24.7)	$ (102.7)	$ (26.0)	$ 27.0	$ 87.6	$ (30.1)	$ 114.6	$ (53.2)	$ 88.6	$ (54.1)	$ (14.1)	$ 57.0
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation												33.2	27.3	31.9
Share-based payments expense												8.7	16.9	18.3
Loss on disposals of long-lived assets, net												1.9	7.2	0.3
Undistributed earnings of subsidiaries												0.0	0.0	0.0
Change in:
Accrued underwriting expenses and other liabilities												(43.7)	(10.6)	111.6
Other, net												9.8	(77.0)	17.8
Cash provided by operating activities												71.9	182.8	301.8
Cash flows from investing activities:
Change in short-term investments												285.4	(490.4)	(132.2)
Settlements of foreign currency exchange forward contracts												9.4	0.3	(1.5)
Capital contribution to subsidiaries												0.0
Proceeds from sale of Ariel Re, net of cash transferred												28.3	0.0	0.0
Dividend received from subsidiaries												0.0
Purchases of fixed assets and other, net												(6.6)	(43.0)	(32.5)
Cash used in investing activities												(24.3)	(142.8)	(268.3)
Cash flows from financing activities:
Issuance of preferred shares, net of issuance costs												144.0	0.0	0.0
Activity under stock incentive plans												1.8	1.9	1.6
Repurchase of Company's common shares												0.0	0.0	(31.7)
Payment of cash dividends to preferred shareholders												(4.6)	0.0	0.0
Payment of cash dividends to common shareholders												(43.0)	(43.1)	(37.5)
Cash used in financing activities												(26.8)	(41.8)	(67.6)
Cash, beginning of year			137.8					137.8		137.8		137.8
Cash, end of period				137.8								148.8	137.8
Argo Group International Holdings, Ltd (Parent  Guarantor)
Cash flows from operating activities:
Net (loss) income												(54.1)	(14.1)	57.0
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation												1.1	1.3	1.4
Share-based payments expense												3.6	8.1	7.0
Net realized investment and other losses (gains)												8.3	0.1	(2.5)
Loss on disposals of long-lived assets, net												0.1	0.0	0.0
Undistributed earnings of subsidiaries												17.4	(23.6)	(74.7)
Change in:
Prepaid assets												(2.2)	2.5	(2.3)
Accrued underwriting expenses and other liabilities												(9.1)	7.3	(4.4)
Due to subsidiaries												47.7	68.9	94.1
Other, net												(7.8)	(12.1)	(3.2)
Cash provided by operating activities												5.0	38.4	72.4
Cash flows from investing activities:
Change in short-term investments												0.0	3.2	(3.4)
Settlements of foreign currency exchange forward contracts												0.1	(0.2)	(0.5)
Capital contribution to subsidiaries												(145.3)	0.0	0.0
Proceeds from sale of Ariel Re, net of cash transferred												30.0	0.0	0.0
Dividend received from subsidiaries												13.1	0.0	0.0
Purchases of fixed assets and other, net												0.0	0.0	(0.1)
Cash used in investing activities												(102.1)	3.0	(4.0)
Cash flows from financing activities:
Issuance of preferred shares, net of issuance costs												144.0	0.0	0.0
Activity under stock incentive plans												1.8	1.9	1.6
Repurchase of Company's common shares												0.0	0.0	(31.7)
Payment of cash dividends to preferred shareholders												(4.6)	0.0	0.0
Payment of cash dividends to common shareholders												(43.0)	(43.1)	(37.5)
Cash used in financing activities												98.2	(41.2)	(67.6)
Change in cash												1.1	0.2	0.8
Cash, beginning of year			$ 1.9				$ 1.7	$ 1.9	$ 1.7	$ 1.9	$ 1.7	1.9	1.7	0.9
Cash, end of period				$ 1.9								$ 3.0	$ 1.9	$ 1.7