Information Provided In Connection With Outstanding Debt Of Subsidiaries (Condensed Consolidating Statement Of Cash Flows) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ 63.5		$ 56.2
Cash flows from investing activities:
Proceeds from sales of investments	898.5		1,053.2
Proceeds from maturities and mandatory calls of investments	364.9		296.7
Purchases of investments	(1,251.4)		(1,326.5)
Change in short-term investments and foreign regulatory deposits	(20.2)		(13.2)
Settlements of foreign currency exchange forward contracts	(0.6)		9.0
Acquisition	(1.4)		(0.5)
Issuance of intercompany note	0		0
Other, net	2.7		1.2
Cash (used) provided by investing activities	(113.2)		30.1
Cash flows from financing activities:
Borrowings under intercompany note	0		0
Proceeds from issuance of senior unsecured fixed rate notes	139.2
Activity under stock incentive plans	0.7		0.9
Repurchase of Company's common shares	(38.6)		(35.9)
Excess tax expense from share-based payment arrangements	0		(0.1)
Payment of cash dividend to common shareholders	(9.3)		(10.0)
Cash provided (used) by financing activities	92.0		(45.1)
Effect of exchange rate changes on cash	(0.5)		(4.9)
Change in cash	41.8		36.3
Cash, beginning of period	100.9		83.5
Cash, end of period	142.7		119.8
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(17.9)		(20.4)
Cash flows from investing activities:
Proceeds from sales of investments	0		0
Proceeds from maturities and mandatory calls of investments	0		0
Purchases of investments	0		0
Change in short-term investments and foreign regulatory deposits	(0.1)		(0.1)
Settlements of foreign currency exchange forward contracts	(0.6)		2.6
Acquisition	0		0
Issuance of intercompany note	0		0
Other, net	0		(0.1)
Cash (used) provided by investing activities	(0.7)		2.4
Cash flows from financing activities:
Borrowings under intercompany note	27.3		27.1
Proceeds from issuance of senior unsecured fixed rate notes	0
Activity under stock incentive plans	0.7		0.9
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		0
Payment of cash dividend to common shareholders	(9.3)		(10.0)
Cash provided (used) by financing activities	18.7		18.0
Effect of exchange rate changes on cash	0		0
Change in cash	0.1		0
Cash, beginning of period	0		0
Cash, end of period	0.1		0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	13.7		14.7
Cash flows from investing activities:
Proceeds from sales of investments	458.9		705.2
Proceeds from maturities and mandatory calls of investments	253.9		190.5
Purchases of investments	(712.1)		(893.3)
Change in short-term investments and foreign regulatory deposits	(64.3)		37.8
Settlements of foreign currency exchange forward contracts	0		0
Acquisition	(1.4)		(0.5)
Issuance of intercompany note	(10.0)		(3.5)
Other, net	(21.6)		(7.4)
Cash (used) provided by investing activities	(96.6)		28.8
Cash flows from financing activities:
Borrowings under intercompany note	25.0		10.0
Proceeds from issuance of senior unsecured fixed rate notes	139.2
Activity under stock incentive plans	0		0
Repurchase of Company's common shares	(38.6)		(35.9)
Excess tax expense from share-based payment arrangements	0		(0.1)
Payment of cash dividend to common shareholders	0		0
Cash provided (used) by financing activities	125.6		(26.0)
Effect of exchange rate changes on cash	0		0
Change in cash	42.7		17.5
Cash, beginning of period	85.1		61.6
Cash, end of period	127.8		79.1
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	40.4		34.8
Cash flows from investing activities:
Proceeds from sales of investments	439.6		348.0
Proceeds from maturities and mandatory calls of investments	111.0		106.2
Purchases of investments	(539.3)		(433.2)
Change in short-term investments and foreign regulatory deposits	(34.6)		(29.9)
Settlements of foreign currency exchange forward contracts	0		6.4
Acquisition	0		0
Issuance of intercompany note	10.0		3.5
Other, net	(2.6)		(2.1)
Cash (used) provided by investing activities	(15.9)		(1.1)
Cash flows from financing activities:
Borrowings under intercompany note	(25.0)		(10.0)
Proceeds from issuance of senior unsecured fixed rate notes	0
Activity under stock incentive plans	0		0
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		0
Payment of cash dividend to common shareholders	0		0
Cash provided (used) by financing activities	(25.0)		(10.0)
Effect of exchange rate changes on cash	(0.5)		(4.9)
Change in cash	(1.0)		18.8
Cash, beginning of period	15.8	[1]	21.9
Cash, end of period	14.8	[1]	40.7
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	27.3		27.1
Cash flows from investing activities:
Proceeds from sales of investments	0		0
Proceeds from maturities and mandatory calls of investments	0		0
Purchases of investments	0		0
Change in short-term investments and foreign regulatory deposits	0		0
Settlements of foreign currency exchange forward contracts	0		0
Acquisition	0		0
Issuance of intercompany note	0		0
Other, net	0		0
Cash (used) provided by investing activities	0		0
Cash flows from financing activities:
Borrowings under intercompany note	(27.3)		(27.1)
Proceeds from issuance of senior unsecured fixed rate notes	0
Activity under stock incentive plans	0		0
Repurchase of Company's common shares	0		0
Excess tax expense from share-based payment arrangements	0		0
Payment of cash dividend to common shareholders	0		0
Cash provided (used) by financing activities	(27.3)		(27.1)
Effect of exchange rate changes on cash	0		0
Change in cash	0		0
Cash, beginning of period	0	[2]	0
Cash, end of period	$ 0	[2]	$ 0

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

[2] Includes all Argo Group parent company eliminations