Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ (103.1)	$ (4.9)
Cash flows from investing activities:
Proceeds from sales of investments	848.3	814.9
Maturities and mandatory calls of fixed maturity investments	255.0	168.8
Purchases of investments	(1,177.4)	(959.1)
Change in short-term investments and foreign regulatory deposits	217.5	74.0
Settlements of foreign currency exchange forward contracts	7.0	9.9
Proceeds from sale of Trident assets	38.0	0.0
Issuance of intercompany note, net	0.0
Purchases of fixed assets and other, net	(12.6)	(38.2)
Cash provided by investing activities	175.8	70.3
Cash flows from financing activities:
Payment on the intercompany note		0.0
Activity under stock incentive plans	1.0	0.9
Payment of cash dividends to common shareholders	(21.3)	(22.7)
Cash used in financing activities	(20.3)	(21.8)
Effect of exchange rate changes on cash	(2.5)	(0.5)
Change in cash	49.9	43.1
Cash, beginning of year	137.8	139.2
Cash, end of period	187.7	182.3
Argo Group International Holdings, Ltd (Parent Guarantor)
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	20.3	19.3
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	(0.1)	2.8
Settlements of foreign currency exchange forward contracts	0.1	(0.1)
Proceeds from sale of Trident assets	0.0
Issuance of intercompany note, net	0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by investing activities	0.0	2.7
Cash flows from financing activities:
Payment on the intercompany note		0.0
Activity under stock incentive plans	1.0	0.9
Payment of cash dividends to common shareholders	(21.3)	(22.7)
Cash used in financing activities	(20.3)	(21.8)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	0.0	0.2
Cash, beginning of year	1.9	1.7
Cash, end of period	1.9	1.9
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(21.0)	(5.5)
Cash flows from investing activities:
Proceeds from sales of investments	454.1	507.1
Maturities and mandatory calls of fixed maturity investments	184.4	131.8
Purchases of investments	(935.0)	(686.4)
Change in short-term investments and foreign regulatory deposits	270.9	74.7
Settlements of foreign currency exchange forward contracts	0.0	0.9
Proceeds from sale of Trident assets	38.0
Issuance of intercompany note, net	(15.0)
Purchases of fixed assets and other, net	33.6	(6.9)
Cash provided by investing activities	31.0	21.2
Cash flows from financing activities:
Payment on the intercompany note		19.1
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	0.0	(19.1)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	10.0	(3.4)
Cash, beginning of year	31.6	31.7
Cash, end of period	41.6	28.3
Other Subsidiaries and Eliminations
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(102.4)	(18.7)
Cash flows from investing activities:
Proceeds from sales of investments	394.2	307.8
Maturities and mandatory calls of fixed maturity investments	70.6	37.0
Purchases of investments	(242.4)	(272.7)
Change in short-term investments and foreign regulatory deposits	(53.3)	(3.5)
Settlements of foreign currency exchange forward contracts	6.9	9.1
Proceeds from sale of Trident assets	0.0
Issuance of intercompany note, net	15.0
Purchases of fixed assets and other, net	(46.2)	(31.3)
Cash provided by investing activities	144.8	46.4
Cash flows from financing activities:
Payment on the intercompany note		(19.1)
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	0.0	19.1
Effect of exchange rate changes on cash	(2.5)	(0.5)
Change in cash	39.9	46.3
Cash, beginning of year	104.3	105.8
Cash, end of period	144.2	152.1
Consolidating Adjustments
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
Proceeds from sale of Trident assets	0.0
Issuance of intercompany note, net	0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by investing activities	0.0	0.0
Cash flows from financing activities:
Payment on the intercompany note		0.0
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	0.0	0.0
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