Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ (4.9)	$ 125.9
Cash flows from investing activities:
Proceeds from sales of investments	814.9	1,025.0
Maturities and mandatory calls of fixed maturity investments	168.8	288.7
Purchases of investments	(959.1)	(1,374.1)
Change in short-term investments and foreign regulatory deposits	74.0	(67.6)
Settlements of foreign currency exchange forward contracts	9.9	2.3
Cash acquired with acquisition of Ariscom		15.6
Purchases of fixed assets and other, net	(38.2)	(27.7)
Cash provided by (used in) investing activities	70.3	(137.8)
Cash flows from financing activities:
Payment on the intercompany note	0.0
Activity under stock incentive plans	0.9	0.2
Repurchase of Company's common shares	0.0	(19.4)
Payment of cash dividends to common shareholders	(22.7)	(18.7)
Cash used in financing activities	(21.8)	(37.9)
Effect of exchange rate changes on cash	(0.5)	(0.1)
Change in cash	43.1	(49.9)
Cash, beginning of year	139.2	176.6
Cash, end of period	182.3	126.7
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	19.3	39.2
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0
Purchases of investments	0.0	0.0
Change in short-term investments and foreign regulatory deposits	2.8	(0.7)
Settlements of foreign currency exchange forward contracts	(0.1)	(0.4)
Cash acquired with acquisition of Ariscom		0.0
Purchases of fixed assets and other, net	0.0	(0.1)
Cash provided by (used in) investing activities	2.7	(1.2)
Cash flows from financing activities:
Payment on the intercompany note	0.0
Activity under stock incentive plans	0.9	0.2
Repurchase of Company's common shares		(19.4)
Payment of cash dividends to common shareholders	(22.7)	(18.7)
Cash used in financing activities	(21.8)	(37.9)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	0.2	0.1
Cash, beginning of year	1.7	0.9
Cash, end of period	1.9	1.0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(5.5)	48.8
Cash flows from investing activities:
Proceeds from sales of investments	507.1	650.8
Maturities and mandatory calls of fixed maturity investments	131.8	226.8
Purchases of investments	(686.4)	(937.5)
Change in short-term investments and foreign regulatory deposits	74.7	(2.8)
Settlements of foreign currency exchange forward contracts	0.9	0.2
Cash acquired with acquisition of Ariscom		0.0
Purchases of fixed assets and other, net	(6.9)	(14.8)
Cash provided by (used in) investing activities	21.2	(77.3)
Cash flows from financing activities:
Payment on the intercompany note	(19.1)
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	(19.1)	0.0
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	(3.4)	(28.5)
Cash, beginning of year	31.7	47.8
Cash, end of period	28.3	19.3
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(18.7)	37.9
Cash flows from investing activities:
Proceeds from sales of investments	307.8	374.2
Maturities and mandatory calls of fixed maturity investments	37.0	61.9
Purchases of investments	(272.7)	(436.6)
Change in short-term investments and foreign regulatory deposits	(3.5)	(64.1)
Settlements of foreign currency exchange forward contracts	9.1	2.5
Cash acquired with acquisition of Ariscom		15.6
Purchases of fixed assets and other, net	(31.3)	(12.8)
Cash provided by (used in) investing activities	46.4	(59.3)
Cash flows from financing activities:
Payment on the intercompany note	19.1
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	19.1	0.0
Effect of exchange rate changes on cash	(0.5)	(0.1)
Change in cash	46.3	(21.5)
Cash, beginning of year	105.8	127.9
Cash, end of period	152.1	106.4
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	0.0	0.0
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0
Purchases of investments	0.0	0.0
Change in short-term investments and foreign regulatory deposits	0.0	0.0
Settlements of foreign currency exchange forward contracts	0.0	0.0
Cash acquired with acquisition of Ariscom		0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by (used in) investing activities	0.0	0.0
Cash flows from financing activities:
Payment on the intercompany note	0.0
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	0.0	0.0
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	0.0	0.0
Cash, beginning of year	0.0	0.0
Cash, end of period	$ 0.0	$ 0.0