Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows provided by operating activities:
Net income (loss)	$ 6.7	$ (54.1)	$ (14.1)
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation	43.4	33.2	27.3
Share-based payments expense	8.0	8.7	16.9
Loss on disposals of long-lived assets, net	23.3	5.5	7.2
Change in:
Accrued underwriting expenses and other liabilities	3.0	(43.7)	(10.6)
Other, net	17.9	9.8	(77.0)
Net cash flows from operating activities	99.7	71.9	182.8
Cash flows used in investing activities:
Change in short-term investments	(116.4)	285.4	(490.4)
Settlements of foreign currency exchange forward contracts	(1.2)	9.4	0.3
Proceeds from sale of Ariel Re, net of cash transferred	0.0	28.3	0.0
Cash used in investing activities	(55.9)	(24.3)	(142.8)
Cash flows from financing activities:
Issuance of preferred shares, net of issuance costs	0.0	144.0	0.0
Activity under stock incentive plans	1.3	1.8	1.9
Payment of cash dividends to preferred shareholders	(10.5)	(4.6)	0.0
Payment of cash dividends to common shareholders	(43.7)	(43.0)	(43.1)
Cash used in financing activities	(52.9)	(26.8)	(41.8)
Cash, beginning of year	148.8
Cash, end of period	146.1	148.8
Argo Group International Holdings, Ltd (Parent  Guarantor)
Cash flows provided by operating activities:
Net income (loss)	6.7	(54.1)	(14.1)
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization and depreciation	1.5	1.1	1.3
Share-based payments expense	2.3	3.6	8.1
Net realized investment and other losses (gains)	0.0	8.3	0.1
Loss on disposals of long-lived assets, net	(3.0)	0.1	0.0
Undistributed (losses) earnings of subsidiaries	(14.3)	17.4	(23.6)
Change in:
Prepaid assets	7.2	(2.2)	2.5
Accrued underwriting expenses and other liabilities	(4.1)	(9.1)	7.3
Due (to) from subsidiaries	(12.1)	(11.1)	16.8
Other, net	(3.9)	(7.8)	(12.1)
Net cash flows from operating activities	(19.7)	(53.8)	(13.7)
Cash flows used in investing activities:
Change in short-term investments	(13.9)	0.0	3.2
Settlements of foreign currency exchange forward contracts	0.5	0.1	(0.2)
Capital contribution to subsidiaries	0.0	(145.3)	0.0
Proceeds from sale of Ariel Re, net of cash transferred	0.0	30.0	0.0
Dividend received from subsidiaries	85.0	71.9	52.1
Cash used in investing activities	71.6	(43.3)	55.1
Cash flows from financing activities:
Issuance of preferred shares, net of issuance costs	0.0	144.0	0.0
Activity under stock incentive plans	1.3	1.8	1.9
Payment of cash dividends to preferred shareholders	(10.5)	(4.6)	0.0
Payment of cash dividends to common shareholders	(43.7)	(43.0)	(43.1)
Cash used in financing activities	(52.9)	98.2	(41.2)
Change in cash	(1.0)	1.1	0.2
Cash, beginning of year	3.0	1.9	1.7
Cash, end of period	$ 2.0	$ 3.0	$ 1.9