Schedule II Condensed Financial Information of Registrant - Schedule of Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net (loss) income	$ (103.3)	$ (25.1)	$ 28.8	$ 91.2	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ (8.4)	$ 63.6	$ 50.3
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									27.3	31.9	33.8
Share-based payments expense									16.9	18.3	12.3
Undistributed earnings of subsidiaries									0.0	0.0	0.0
Change in:
Accrued underwriting expenses and other liabilities									(5.4)	110.2	(23.5)
Other, net									(79.9)	18.2	(1.5)
Cash provided by operating activities									183.3	301.3	165.0
Cash flows from investing activities:
Change in short-term investments									(490.4)	(132.2)	306.7
Settlements of foreign currency exchange forward contracts									0.3	(1.5)	(2.9)
Acquisition of subsidiaries, net of cash									0.0	0.0	(105.2)
Purchases of fixed assets and other, net									(43.0)	(32.5)	(60.1)
Cash used in investing activities									(142.8)	(268.3)	(121.3)
Cash flows from financing activities:
Activity under stock incentive plans									1.9	1.6	1.4
Repurchase of Company's common shares									0.0	(31.7)	(45.2)
Payment of cash dividends to common shareholders									(43.1)	(37.5)	(33.2)
Cash (used in) provided by financing activities									(41.8)	(67.6)	48.0
Cash, beginning of year				139.2					139.2
Cash, end of period	137.8				139.2				137.8	139.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Cash flows from operating activities:
Net (loss) income									(8.4)	63.6	50.3
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									1.3	1.4	1.3
Share-based payments expense									8.1	7.0	3.3
Net realized investment and other gains									0.1	(2.5)	(0.4)
Undistributed earnings of subsidiaries									22.8	(44.8)	(73.1)
Change in:
Prepaid assets									2.5	(2.3)	(0.3)
Accrued underwriting expenses and other liabilities									7.3	(4.4)	(2.3)
Due to subsidiaries									16.8	57.6	48.1
Other, net									(12.1)	(3.2)	(6.1)
Cash provided by operating activities									38.4	72.4	20.8
Cash flows from investing activities:
Change in short-term investments									3.2	(3.4)	1.5
Settlements of foreign currency exchange forward contracts									(0.2)	(0.5)	0.9
Acquisition of subsidiaries, net of cash									0.0	0.0	(235.3)
Purchases of fixed assets and other, net									0.0	(0.1)	(0.1)
Cash used in investing activities									3.0	(4.0)	(233.0)
Cash flows from financing activities:
Additional borrowings									0.0	0.0	125.0
Borrowings under intercompany note payable, net									0.0	0.0	120.0
Activity under stock incentive plans									1.9	1.6	1.4
Repurchase of Company's common shares									0.0	(31.7)	(0.1)
Payment of cash dividends to common shareholders									(43.1)	(37.5)	(33.2)
Cash (used in) provided by financing activities									(41.2)	(67.6)	213.1
Change in cash									0.2	0.8	0.9
Cash, beginning of year				$ 1.7				$ 0.9	1.7	0.9	0.0
Cash, end of period	$ 1.9				$ 1.7				$ 1.9	$ 1.7	$ 0.9