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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ (43.6)	$ 40.6	$ 41.8	$ 24.8	$ 28.9	$ (61.3)	$ 46.0	$ 36.7	$ 63.6	$ 50.3	$ 146.7
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									31.9	33.8	35.4
Share-based payments expense									18.3	12.3	19.8
Undistributed earnings of subsidiaries									0.0	0.0	0.0
Change in:
Accrued underwriting expenses									15.2	(24.4)	(18.9)
Other, net									113.2	(0.6)	(48.9)
Cash provided by operating activities									301.3	165.0	182.0
Cash flows from investing activities:
Change in short-term investments									(132.2)	306.7	(193.9)
Settlements of foreign currency exchange forward contracts									(1.5)	(2.9)	(5.4)
Acquisition of subsidiaries, net of cash										(105.2)
Purchases of fixed assets and other, net									(32.5)	(60.1)	(10.2)
Cash used in investing activities									(268.3)	(121.3)	(145.1)
Cash flows from financing activities:
Activity under stock incentive plans									1.6	1.4	1.0
Repurchase of Company's common shares									(31.7)	(45.2)	(47.1)
Payment of cash dividends to common shareholders									(37.5)	(33.2)	(26.6)
Cash (used in) provided by financing activities									(67.6)	48.0	(72.7)
Change in cash									(37.4)	90.6	(35.7)
Cash, beginning of year				176.6				86.0	176.6	86.0	121.7
Cash, end of period	139.2				176.6				139.2	176.6	86.0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Cash flows from operating activities:
Net income									63.6	50.3	146.7
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Amortization and depreciation									1.4	1.3	0.2
Share-based payments expense									7.0	3.3	4.2
Net realized investment and other gains									(2.5)	(0.4)	(0.6)
Undistributed earnings of subsidiaries									(44.8)	(73.1)	(123.1)
Change in:
Prepaid assets									(2.3)	(0.3)	(0.2)
Accrued underwriting expenses									(4.4)	(2.3)	0.9
Due to subsidiaries									57.6	48.1	(0.6)
Other, net									(3.2)	(6.1)	(1.0)
Cash provided by operating activities									72.4	20.8	26.5
Cash flows from investing activities:
Change in short-term investments									(3.4)	1.5	(0.9)
Settlements of foreign currency exchange forward contracts									(0.5)	0.9	0.0
Acquisition of subsidiaries, net of cash									0.0	(235.3)	0.0
Purchases of fixed assets and other, net									(0.1)	(0.1)	0.0
Cash used in investing activities									(4.0)	(233.0)	(0.9)
Cash flows from financing activities:
Additional borrowings									0.0	125.0	0.0
Borrowings under intercompany note payable, net									0.0	120.0	0.0
Activity under stock incentive plans									1.6	1.4	1.0
Repurchase of Company's common shares									(31.7)	(0.1)	0.0
Payment of cash dividends to common shareholders									(37.5)	(33.2)	(26.6)
Cash (used in) provided by financing activities									(67.6)	213.1	(25.6)
Change in cash									0.8	0.9	0.0
Cash, beginning of year				$ 0.9				$ 0.0	0.9	0.0	0.0
Cash, end of period	$ 1.7				$ 0.9				$ 1.7	$ 0.9	$ 0.0