Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Borrowing under intercompany note, net		$ 0.0
Net cash flows from operating activities	$ 195.7	231.0
Cash flows from investing activities:
Proceeds from sales of investments	1,180.3	1,417.2
Maturities and mandatory calls of fixed maturity investments	292.2	375.1
Purchases of investments	(1,639.5)	(1,977.2)
Change in short-term investments and foreign regulatory deposits	2.3	(33.8)
Settlements of foreign currency exchange forward contracts	(1.2)	2.0
Cash acquired with acquisition of Ariscom	0.0	15.6
Purchases of fixed assets and other, net	6.7	(32.2)
Cash used in investing activities	(159.2)	(233.3)
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	(0.6)	0.0
Activity under stock incentive plans	1.4	1.0
Repurchase of Company's common shares	0.0	(30.0)
Payment of cash dividends to common shareholders	(32.5)	(28.0)
Cash used in financing activities	(31.7)	(57.0)
Effect of exchange rate changes on cash	1.1	1.2
Change in cash	5.9	(58.1)
Cash, beginning of year	139.2	176.6
Cash, end of period	145.1	118.5
Argo Group International Holdings, Ltd (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Borrowing under intercompany note, net		0.0
Net cash flows from operating activities	29.2	57.7
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	3.5	0.0
Settlements of foreign currency exchange forward contracts	(0.1)	0.2
Cash acquired with acquisition of Ariscom		0.0
Purchases of fixed assets and other, net	0.0	(0.1)
Cash used in investing activities	3.4	0.1
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	0.0
Activity under stock incentive plans	1.4	1.0
Repurchase of Company's common shares		(30.0)
Payment of cash dividends to common shareholders	(32.5)	(28.0)
Cash used in financing activities	(31.1)	(57.0)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	1.5	0.8
Cash, beginning of year	1.7	0.9
Cash, end of period	3.2	1.7
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Borrowing under intercompany note, net		19.0
Net cash flows from operating activities	124.6	151.3
Cash flows from investing activities:
Proceeds from sales of investments	744.2	963.2
Maturities and mandatory calls of fixed maturity investments	219.2	302.5
Purchases of investments	(1,091.2)	(1,346.8)
Change in short-term investments and foreign regulatory deposits	34.7	(65.0)
Settlements of foreign currency exchange forward contracts	1.4	0.5
Cash acquired with acquisition of Ariscom		0.0
Purchases of fixed assets and other, net	(1.8)	(38.5)
Cash used in investing activities	(93.5)	(184.1)
Cash flows from financing activities:
Payment on the intercompany note	(19.1)
Payment on note payable	(0.6)
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	(19.7)	19.0
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	11.4	(13.8)
Cash, beginning of year	31.7	47.8
Cash, end of period	43.1	34.0
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Borrowing under intercompany note, net		(19.0)
Net cash flows from operating activities	41.9	22.0
Cash flows from investing activities:
Proceeds from sales of investments	436.1	454.0
Maturities and mandatory calls of fixed maturity investments	73.0	72.6
Purchases of investments	(548.3)	(630.4)
Change in short-term investments and foreign regulatory deposits	(35.9)	31.2
Settlements of foreign currency exchange forward contracts	(2.5)	1.3
Cash acquired with acquisition of Ariscom		15.6
Purchases of fixed assets and other, net	8.5	6.4
Cash used in investing activities	(69.1)	(49.3)
Cash flows from financing activities:
Payment on the intercompany note	19.1
Payment on note payable	0.0
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash used in financing activities	19.1	(19.0)
Effect of exchange rate changes on cash	1.1	1.2
Change in cash	(7.0)	(45.1)
Cash, beginning of year	105.8	127.9
Cash, end of period	98.8	82.8
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Borrowing under intercompany note, net		0.0
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
Purchases of fixed assets and other, net	0.0	0.0
Cash used in investing activities	0.0	0.0
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	0.0
Activity under stock incentive plans	0.0	0.0
Repurchase of Company's common shares		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