Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ 125.9		$ 37.3
Cash flows from investing activities:
Proceeds from sales of investments	1,025.0		958.4
Maturities and mandatory calls of fixed maturity investments	288.7		335.1
Purchases of investments	(1,374.1)		(1,466.0)
Change in short-term investments and foreign regulatory deposits	(67.6)		300.4
Settlements of foreign currency exchange forward contracts	2.3		0.5
Cash acquired with acquisition of Ariscom	15.6
Acquisition of Maybrooke, net of cash acquired			(105.2)
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	(27.7)		(37.7)
Cash used in investing activities	(137.8)		(14.5)
Cash flows from financing activities:
Additional long-term borrowings	0.0		125.0
Borrowing under intercompany note, net			0.0
Activity under stock incentive plans	0.2		0.5
Repurchase of Company's common shares	(19.4)		(2.8)
Payment of cash dividends to common shareholders	(18.7)		(16.6)
Cash (used in) provided by financing activities	(37.9)		106.1
Effect of exchange rate changes on cash	(0.1)		(0.4)
Change in cash	(49.9)		128.5
Cash, beginning of year	176.6	[1]	86.0
Cash, end of period	126.7		214.5
Argo Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	39.2		5.3
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	(0.7)		0.4
Settlements of foreign currency exchange forward contracts	(0.4)		0.8
Cash acquired with acquisition of Ariscom	0.0
Acquisition of Maybrooke, net of cash acquired			(235.3)
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	(0.1)		(0.1)
Cash used in investing activities	(1.2)		(234.2)
Cash flows from financing activities:
Additional long-term borrowings			125.0
Borrowing under intercompany note, net			120.0
Activity under stock incentive plans	0.2		0.5
Repurchase of Company's common shares	(19.4)		0.0
Payment of cash dividends to common shareholders	(18.7)		(16.6)
Cash (used in) provided by financing activities	(37.9)		228.9
Effect of exchange rate changes on cash	0.0		0.0
Change in cash	0.1		0.0
Cash, beginning of year	0.9		0.0
Cash, end of period	1.0		0.0
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	48.8		(8.0)
Cash flows from investing activities:
Proceeds from sales of investments	650.8		514.4
Maturities and mandatory calls of fixed maturity investments	226.8		240.6
Purchases of investments	(937.5)		(811.0)
Change in short-term investments and foreign regulatory deposits	(2.8)		72.0
Settlements of foreign currency exchange forward contracts	0.2		0.0
Cash acquired with acquisition of Ariscom	0.0
Acquisition of Maybrooke, net of cash acquired			0.0
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	(14.8)		(12.7)
Cash used in investing activities	(77.3)		3.3
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	0.0		(2.8)
Payment of cash dividends to common shareholders	0.0		0.0
Cash (used in) provided by financing activities	0.0		(2.8)
Effect of exchange rate changes on cash	0.0		0.0
Change in cash	(28.5)		(7.5)
Cash, beginning of year	47.8		53.7
Cash, end of period	19.3		46.2
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	37.9		40.0
Cash flows from investing activities:
Proceeds from sales of investments	374.2		444.0
Maturities and mandatory calls of fixed maturity investments	61.9		94.5
Purchases of investments	(436.6)		(655.0)
Change in short-term investments and foreign regulatory deposits	(64.1)		228.0
Settlements of foreign currency exchange forward contracts	2.5		(0.3)
Cash acquired with acquisition of Ariscom	15.6
Acquisition of Maybrooke, net of cash acquired			130.1
Issuance of intercompany note, net			(120.0)
Purchases of fixed assets and other, net	(12.8)		(24.9)
Cash used in investing activities	(59.3)		96.4
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	0.0		0.0
Payment of cash dividends to common shareholders	0.0		0.0
Cash (used in) provided by financing activities	0.0		0.0
Effect of exchange rate changes on cash	(0.1)		(0.4)
Change in cash	(21.5)		136.0
Cash, beginning of year	127.9		32.3
Cash, end of period	106.4		168.3
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
Acquisition of Maybrooke, net of cash acquired			0.0
Issuance of intercompany note, net			120.0
Purchases of fixed assets and other, net	0.0		0.0
Cash used in investing activities	0.0		120.0
Cash flows from financing activities:
Additional long-term borrowings			0.0
Borrowing under intercompany note, net			(120.0)
Activity under stock incentive plans	0.0		0.0
Repurchase of Company's common shares	0.0		0.0
Payment of cash dividends to common shareholders	0.0		0.0
Cash (used in) provided by financing activities	0.0		(120.0)
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

[1] Derived from audited consolidated financial statements.