Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ (31.4)	$ 55.6
Cash flows from investing activities:
Proceeds from sales of investments	638.1	272.5
Maturities and mandatory calls of fixed maturity investments	158.2	65.8
Purchases of investments	(849.2)	(384.2)
Change in short-term investments and foreign regulatory deposits	116.2	38.0
Settlements of foreign currency exchange forward contracts	3.1	3.6
Purchases of fixed assets and other, net	(5.1)	(27.4)
Cash provided by (used in) investing activities	61.3	(31.7)
Cash flows from financing activities:
Payment on the intercompany note		0.0
Activity under stock incentive plans	0.3	0.3
Payment of cash dividends to common shareholders	(10.7)	(10.8)
Cash used in financing activities	(10.4)	(10.5)
Effect of exchange rate changes on cash	1.4	0.1
Change in cash	20.9	13.5
Cash, beginning of year	137.8	139.2
Cash, end of period	158.7	152.7
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	10.0	8.8
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	0.3	1.9
Settlements of foreign currency exchange forward contracts	0.0	0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by (used in) investing activities	0.3	1.9
Cash flows from financing activities:
Payment on the intercompany note		0.0
Activity under stock incentive plans	0.3	0.3
Payment of cash dividends to common shareholders	(10.7)	(10.8)
Cash used in financing activities	(10.4)	(10.5)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	(0.1)	0.2
Cash, beginning of year	1.9	1.7
Cash, end of period	1.8	1.9
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	35.7	29.5
Cash flows from investing activities:
Proceeds from sales of investments	309.4	156.6
Maturities and mandatory calls of fixed maturity investments	100.5	51.3
Purchases of investments	(583.0)	(249.0)
Change in short-term investments and foreign regulatory deposits	128.8	35.3
Settlements of foreign currency exchange forward contracts	0.0	0.3
Purchases of fixed assets and other, net	17.4	(3.3)
Cash provided by (used in) investing activities	(26.9)	(8.8)
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	8.8	1.6
Cash, beginning of year	31.6	31.7
Cash, end of period	40.4	33.3
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	(77.1)	17.3
Cash flows from investing activities:
Proceeds from sales of investments	328.7	115.9
Maturities and mandatory calls of fixed maturity investments	57.7	14.5
Purchases of investments	(266.2)	(135.2)
Change in short-term investments and foreign regulatory deposits	(12.9)	0.8
Settlements of foreign currency exchange forward contracts	3.1	3.3
Purchases of fixed assets and other, net	(22.5)	(24.1)
Cash provided by (used in) investing activities	87.9	(24.8)
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	1.4	0.1
Change in cash	12.2	11.7
Cash, beginning of year	104.3	105.8
Cash, end of period	116.5	117.5
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
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by (used in) 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