Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		$ 252.4		$ 150.8
Cash flows from investing activities:
Proceeds from sales of investments		1,278.0		1,009.2
Maturities and mandatory calls of fixed maturity investments		493.2		839.0
Purchases of investments		(2,060.2)		(1,855.7)
Change in short-term investments and foreign regulatory deposits		264.2		(73.9)
Settlements of foreign currency exchange forward contracts		0.0		(7.2)
Acquisition of subsidiaries, net of cash		(105.2)		0.0
Issuance of intercompany note, net		0.0
Purchases of fixed assets and other, net		(36.0)		5.2
Cash used in investing activities		(166.0)		(83.4)
Cash flows from financing activities:
Additional long-term borrowings		125.0		0.0
Borrowing under intercompany note, net		0.0
Activity under stock incentive plans		0.9		0.6
Repurchase of Company's common shares		(36.6)		(45.3)
Payment of cash dividends to common shareholders		(24.9)		(19.8)
Cash provided by (used in) financing activities		64.4		(64.5)
Effect of exchange rate changes on cash		(1.0)		(1.0)
Change in cash		149.8		1.9
Cash, beginning of year		86.0	[1]	121.7
Cash, end of period		235.8		123.6
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		11.8		19.6
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		1.8		(0.4)
Settlements of foreign currency exchange forward contracts		0.8		0.0
Acquisition of subsidiaries, net of cash		(235.3)
Issuance of intercompany note, net		0.0
Purchases of fixed assets and other, net		(0.1)		0.0
Cash used in investing activities		(232.8)		(0.4)
Cash flows from financing activities:
Additional long-term borrowings		125.0
Borrowing under intercompany note, net		120.0
Activity under stock incentive plans		0.9		0.6
Repurchase of Company's common shares		0.0		0.0
Payment of cash dividends to common shareholders		(24.9)		(19.8)
Cash provided by (used in) financing activities		221.0		(19.2)
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
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities		175.3		60.7
Cash flows from investing activities:
Proceeds from sales of investments		652.5		762.4
Maturities and mandatory calls of fixed maturity investments		350.6		419.7
Purchases of investments		(1,140.8)		(1,134.6)
Change in short-term investments and foreign regulatory deposits		29.6		(70.5)
Settlements of foreign currency exchange forward contracts		(5.5)		0.0
Acquisition of subsidiaries, net of cash		0.0
Issuance of intercompany note, net		0.0
Purchases of fixed assets and other, net		(5.3)		8.1
Cash used in investing activities		(118.9)		(14.9)
Cash flows from financing activities:
Additional long-term borrowings		0.0
Borrowing under intercompany note, net		0.0
Activity under stock incentive plans		0.0		0.0
Repurchase of Company's common shares		(36.6)		(45.3)
Payment of cash dividends to common shareholders		0.0		0.0
Cash provided by (used in) financing activities		(36.6)		(45.3)
Effect of exchange rate changes on cash		0.0		0.0
Change in cash		19.8		0.5
Cash, beginning of year		53.7		88.8
Cash, end of period		73.5		89.3
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	[2]	65.3		70.5
Cash flows from investing activities:
Proceeds from sales of investments	[2]	625.5		246.8
Maturities and mandatory calls of fixed maturity investments	[2]	142.6		419.3
Purchases of investments	[2]	(919.4)		(721.1)
Change in short-term investments and foreign regulatory deposits	[2]	232.8		(3.0)
Settlements of foreign currency exchange forward contracts	[2]	4.7		(7.2)
Acquisition of subsidiaries, net of cash	[2]	130.1
Issuance of intercompany note, net	[2]	(120.0)
Purchases of fixed assets and other, net	[2]	(30.6)		(2.9)
Cash used in investing activities	[2]	65.7		(68.1)
Cash flows from financing activities:
Additional long-term borrowings	[2]	0.0
Borrowing under intercompany note, net	[2]	0.0
Activity under stock incentive plans	[2]	0.0		0.0
Repurchase of Company's common shares	[2]	0.0		0.0
Payment of cash dividends to common shareholders	[2]	0.0		0.0
Cash provided by (used in) financing activities	[2]	0.0		0.0
Effect of exchange rate changes on cash	[2]	(1.0)		(1.0)
Change in cash	[2]	130.0		1.4
Cash, beginning of year	[2]	32.3		32.9
Cash, end of period	[2]	162.3		34.3
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
Acquisition of subsidiaries, net of cash	[3]	0.0
Issuance of intercompany note, net	[3]	120.0
Purchases of fixed assets and other, net	[3]	0.0		0.0
Cash used in investing activities	[3]	120.0		0.0
Cash flows from financing activities:
Additional long-term borrowings	[3]	0.0
Borrowing under intercompany note, net	[3]	(120.0)
Activity under stock incentive plans	[3]	0.0		0.0
Repurchase of Company's common shares	[3]	0.0		0.0
Payment of cash dividends to common shareholders	[3]	0.0		0.0
Cash provided by (used in) financing activities	[3]	(120.0)		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 year	[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.