Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		$ 150.7		$ 235.8
Cash flows from investing activities:
Proceeds from sales of investments		1,009.2		728.8
Maturities and mandatory calls of fixed maturity investments		839.0		637.4
Purchases of investments		(1,855.7)		(1,468.5)
Change in short-term investments and foreign regulatory deposits		(73.9)		(62.4)
Settlements of foreign currency exchange forward contracts		(7.2)		(6.7)
Purchases of fixed assets		(26.7)		(22.0)
Issuance of intercompany note, net				0.0
Purchases of fixed assets and other, net		5.2		(4.6)
Cash used in investing activities		(83.4)		(176.0)
Cash flows from financing activities:
Activity under stock incentive plans		0.6		1.5
Repurchase of Company's common shares		(45.3)		(29.7)
Excess tax expense from share-based payment arrangements		0.1		0.6
Payment of cash dividends to common shareholders		(19.8)		(17.1)
Cash used in financing activities		(64.4)		(44.7)
Effect of exchange rate changes on cash		(1.0)		0.1
Change in cash		1.9		15.2
Cash, beginning of period		121.7	[1]	81.0
Cash, end of period		123.6		96.2
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		19.6		12.6
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		(0.4)		1.3
Settlements of foreign currency exchange forward contracts		0.0		1.5
Issuance of intercompany note, net				0.0
Purchases of fixed assets and other, net		0.0		0.2
Cash used in investing activities		(0.4)		3.0
Cash flows from financing activities:
Activity under stock incentive plans		0.6		1.5
Repurchase of Company's common shares		0.0		0.0
Excess tax expense from share-based payment arrangements		0.0		0.0
Payment of cash dividends to common shareholders		(19.8)		(17.1)
Cash used in financing activities		(19.2)		(15.6)
Effect of exchange rate changes on cash		0.0		0.0
Change in cash		0.0		0.0
Cash, beginning of period		0.0		0.0
Cash, end of period		0.0		0.0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		60.6		130.6
Cash flows from investing activities:
Proceeds from sales of investments		762.4		524.5
Maturities and mandatory calls of fixed maturity investments		419.7		509.5
Purchases of investments		(1,134.6)		(1,073.2)
Change in short-term investments and foreign regulatory deposits		(70.5)		(34.5)
Settlements of foreign currency exchange forward contracts		0.0		0.0
Issuance of intercompany note, net				15.0
Purchases of fixed assets and other, net		8.1		(7.3)
Cash used in investing activities		(14.9)		(66.0)
Cash flows from financing activities:
Activity under stock incentive plans		0.0		0.0
Repurchase of Company's common shares		(45.3)		(29.7)
Excess tax expense from share-based payment arrangements		0.1		0.6
Payment of cash dividends to common shareholders		0.0		0.0
Cash used in financing activities		(45.2)		(29.1)
Effect of exchange rate changes on cash		0.0		0.0
Change in cash		0.5		35.5
Cash, beginning of period		88.8		49.3
Cash, end of period		89.3		84.8
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[2]	70.5		92.6
Cash flows from investing activities:
Proceeds from sales of investments	[2]	246.8		204.3
Maturities and mandatory calls of fixed maturity investments	[2]	419.3		127.9
Purchases of investments	[2]	(721.1)		(395.3)
Change in short-term investments and foreign regulatory deposits	[2]	(3.0)		(29.2)
Settlements of foreign currency exchange forward contracts	[2]	(7.2)		(8.2)
Issuance of intercompany note, net	[2]			(15.0)
Purchases of fixed assets and other, net	[2]	(2.9)		2.5
Cash used in investing activities	[2]	(68.1)		(113.0)
Cash flows from financing activities:
Activity under stock incentive plans	[2]	0.0		0.0
Repurchase of Company's common shares	[2]	0.0		0.0
Excess tax expense from share-based payment arrangements	[2]	0.0		0.0
Payment of cash dividends to common shareholders	[2]	0.0		0.0
Cash used in financing activities	[2]	0.0		0.0
Effect of exchange rate changes on cash	[2]	(1.0)		0.1
Change in cash	[2]	1.4		(20.3)
Cash, beginning of period	[2]	32.9		31.7
Cash, end of period	[2]	34.3		11.4
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[3]	0.0		0.0
Cash flows from investing activities:
Proceeds from sales of investments	[3]	0.0		0.0
Maturities and mandatory calls of fixed maturity investments	[3]	0.0		0.0
Purchases of investments	[3]	0.0		0.0
Change in short-term investments and foreign regulatory deposits	[3]	0.0		0.0
Settlements of foreign currency exchange forward contracts	[3]	0.0		0.0
Issuance of intercompany note, net	[3]			0.0
Purchases of fixed assets and other, net	[3]	0.0		0.0
Cash used in investing activities	[3]	0.0		0.0
Cash flows from financing activities:
Activity under stock incentive plans	[3]	0.0		0.0
Repurchase of Company's common shares	[3]	0.0		0.0
Excess tax expense from share-based payment arrangements	[3]	0.0		0.0
Payment of cash dividends to common shareholders	[3]	0.0		0.0
Cash used in financing activities	[3]	0.0		0.0
Effect of exchange rate changes on cash	[3]	0.0		0.0
Change in cash	[3]	0.0		0.0
Cash, beginning of period	[3]	0.0		0.0
Cash, end of period	[3]	$ 0.0		$ 0.0

[1] Derived from audited consolidated financial statements.

[2] Includes all other subsidiaries of Argo Group International Holdings, Ltd. and all intercompany eliminations.

[3] Includes all Argo Group parent company eliminations.