Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		$ 181.4	$ 282.6	$ 130.5
Cash flows from investing activities:
Proceeds from sales of investments		1,443.5	967.9	1,262.0
Maturities and mandatory calls of fixed maturity investments		1,002.7	843.9	323.0
Purchases of investments		(2,380.5)	(2,034.1)	(1,736.8)
Change in short-term investments and foreign regulatory deposits		(195.2)	49.6	96.5
Settlements of foreign currency exchange forward contracts		(5.4)	(10.1)	(1.1)
Issuance of intercompany note, net			0.0	0.0
Redemption of PXRE Capital Trust V			0.0
Purchases of fixed assets and other, net		(10.2)	(10.8)	(64.9)
Cash used in investing activities		(145.1)	(193.6)	(121.3)
Cash flows from financing activities:
Borrowings under intercompany note, net		0.0		0.0
Activity under stock incentive plans		1.0	1.8	4.6
Redemption of trust preferred securities, net		0.0	0.0	(18.0)
Payment on note payable		0.0	0.0	(0.1)
Repurchase of Company's common shares		(47.1)	(29.7)	(50.8)
Excess tax benefit from share-based payment arrangements		0.6	0.6	0.1
Payment of cash dividends to common shareholders		(26.6)	(22.7)	(18.2)
Intercompany cash dividend		0.0
Cash used in financing activities		(72.1)	(50.0)	(82.4)
Effect of exchange rate changes on cash		0.1	1.7	(3.2)
Change in cash		(35.7)	40.7	(76.4)
Cash, beginning of year		121.7	81.0	157.4
Cash, end of year		86.0	121.7	81.0
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		26.5	32.7	25.7
Cash flows from investing activities:
Proceeds from sales of investments		0.0	0.0	0.0
Maturities and mandatory calls of fixed maturity investments		0.0	0.0	0.0
Purchases of investments		0.0	0.0	0.0
Change in short-term investments and foreign regulatory deposits		(0.9)	0.9	0.5
Settlements of foreign currency exchange forward contracts		0.0	1.5	1.3
Issuance of intercompany note, net			0.0	0.0
Redemption of PXRE Capital Trust V			0.0
Purchases of fixed assets and other, net		0.0	3.8	(7.0)
Cash used in investing activities		(0.9)	6.2	(5.2)
Cash flows from financing activities:
Borrowings under intercompany note, net		0.0		(6.9)
Activity under stock incentive plans		1.0	1.8	4.6
Redemption of trust preferred securities, net		0.0	(18.0)	0.0
Payment on note payable		0.0		0.0
Repurchase of Company's common shares		0.0	0.0	0.0
Excess tax benefit from share-based payment arrangements		0.0	0.0	0.0
Payment of cash dividends to common shareholders		(26.6)	(22.7)	(18.2)
Intercompany cash dividend		0.0
Cash used in financing activities		(25.6)	(38.9)	(20.5)
Effect of exchange rate changes on cash		0.0	0.0	0.0
Change in cash		0.0	0.0	0.0
Cash, beginning of year		0.0	0.0	0.0
Cash, end of year		0.0	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		71.7	116.4	43.2
Cash flows from investing activities:
Proceeds from sales of investments		1,035.9	631.1	803.8
Maturities and mandatory calls of fixed maturity investments		543.2	681.8	192.1
Purchases of investments		(1,450.0)	(1,384.5)	(1,126.1)
Change in short-term investments and foreign regulatory deposits		(138.1)	14.9	76.0
Settlements of foreign currency exchange forward contracts		0.0	0.0	0.0
Issuance of intercompany note, net			7.5	14.5
Redemption of PXRE Capital Trust V			18.0
Purchases of fixed assets and other, net		(11.3)	(16.6)	(35.5)
Cash used in investing activities		(20.3)	(47.8)	(75.2)
Cash flows from financing activities:
Borrowings under intercompany note, net		0.0		0.0
Activity under stock incentive plans		0.0	0.0	0.0
Redemption of trust preferred securities, net		0.0	0.0	0.0
Payment on note payable		0.0		(0.1)
Repurchase of Company's common shares		(47.1)	(29.7)	(50.8)
Excess tax benefit from share-based payment arrangements		0.6	0.6	0.1
Payment of cash dividends to common shareholders		0.0	0.0	0.0
Intercompany cash dividend		(40.0)
Cash used in financing activities		(86.5)	(29.1)	(50.8)
Effect of exchange rate changes on cash		0.0	0.0	0.0
Change in cash		(35.1)	39.5	(82.8)
Cash, beginning of year		88.8	49.3	132.1
Cash, end of year		53.7	88.8	49.3
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[1]	83.2	133.5	37.2
Cash flows from investing activities:
Proceeds from sales of investments	[1]	407.6	336.8	458.2
Maturities and mandatory calls of fixed maturity investments	[1]	459.5	162.1	130.9
Purchases of investments	[1]	(930.5)	(649.6)	(610.7)
Change in short-term investments and foreign regulatory deposits	[1]	(56.2)	33.8	20.0
Settlements of foreign currency exchange forward contracts	[1]	(5.4)	(11.6)	(2.4)
Issuance of intercompany note, net	[1]		(7.5)	(7.6)
Redemption of PXRE Capital Trust V	[1]		(18.0)
Purchases of fixed assets and other, net	[1]	1.1	2.0	(16.0)
Cash used in investing activities	[1]	(123.9)	(152.0)	(27.6)
Cash flows from financing activities:
Borrowings under intercompany note, net	[1]	0.0		0.0
Activity under stock incentive plans	[1]	0.0	0.0	0.0
Redemption of trust preferred securities, net	[1]	0.0	18.0	0.0
Payment on note payable	[1]	0.0		0.0
Repurchase of Company's common shares	[1]	0.0	0.0	0.0
Excess tax benefit from share-based payment arrangements	[1]	0.0	0.0	0.0
Payment of cash dividends to common shareholders	[1]	0.0	0.0	0.0
Intercompany cash dividend	[1]	40.0
Cash used in financing activities	[1]	40.0	18.0	0.0
Effect of exchange rate changes on cash	[1]	0.1	1.7	(3.2)
Change in cash	[1]	(0.6)	1.2	6.4
Cash, beginning of year	[1]	32.9	31.7	25.3
Cash, end of year	[1]	32.3	32.9	31.7
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[2]	0.0	0.0	24.4
Cash flows from investing activities:
Proceeds from sales of investments	[2]	0.0	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	[2]	0.0	0.0	0.0
Purchases of investments	[2]	0.0	0.0	0.0
Change in short-term investments and foreign regulatory deposits	[2]	0.0	0.0	0.0
Settlements of foreign currency exchange forward contracts	[2]	0.0	0.0	0.0
Issuance of intercompany note, net	[2]		0.0	(6.9)
Redemption of PXRE Capital Trust V	[2]		0.0
Purchases of fixed assets and other, net	[2]	0.0	0.0	(6.4)
Cash used in investing activities	[2]	0.0	0.0	(13.3)
Cash flows from financing activities:
Borrowings under intercompany note, net	[2]	0.0		6.9
Activity under stock incentive plans	[2]	0.0	0.0	0.0
Redemption of trust preferred securities, net	[2]	0.0	0.0	(18.0)
Payment on note payable	[2]	0.0		0.0
Repurchase of Company's common shares	[2]	0.0	0.0	0.0
Excess tax benefit from share-based payment arrangements	[2]	0.0	0.0	0.0
Payment of cash dividends to common shareholders	[2]	0.0	0.0	0.0
Intercompany cash dividend	[2]	0.0
Cash used in financing activities	[2]	0.0	0.0	(11.1)
Effect of exchange rate changes on cash	[2]	0.0	0.0	0.0
Change in cash	[2]	0.0	0.0	0.0
Cash, beginning of year	[2]	0.0	0.0	0.0
Cash, end of year	[2]	$ 0.0	$ 0.0	$ 0.0

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

[2] Includes all Argo Group parent company eliminations.