Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ 183.3	$ 301.3	$ 165.0
Cash flows from investing activities:
Proceeds from sales of investments	1,852.1	1,599.8	1,729.9
Maturities and mandatory calls of fixed maturity investments	522.2	418.6	678.3
Purchases of investments	(1,984.0)	(2,149.1)	(2,660.8)
Change in short-term investments and foreign regulatory deposits	(490.4)	(119.2)	299.5
Settlements of foreign currency exchange forward contracts	0.3	(1.5)	(2.9)
Cash acquired with acquisition of Ariscom	0.0	15.6	0.0
Acquisition of subsidiaries, net of cash	0.0	0.0	(105.2)
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	(43.0)	(32.5)	(60.1)
Cash used in investing activities	(142.8)	(268.3)	(121.3)
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	(0.6)	0.0	0.0
Additional long-term borrowings	0.0	0.0	125.0
Borrowing under the intercompany note		0.0	0.0
Activity under stock incentive plans	1.9	1.6	1.4
Repurchase of Company's common shares	0.0	(31.7)	(45.2)
Payment of cash dividends to common shareholders	(43.1)	(37.5)	(33.2)
Cash (used in) provided by financing activities	(41.8)	(67.6)	48.0
Effect of exchange rate changes on cash	(0.1)	(2.8)	(1.1)
Change in cash	(1.4)	(37.4)	90.6
Cash, beginning of year	139.2	176.6	86.0
Cash, end of period	137.8	139.2	176.6
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	0.0	0.0	0.0
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0	0.0
Purchases of investments	0.0	0.0	0.0
Change in short-term investments and foreign regulatory deposits	0.0	0.0	0.0
Settlements of foreign currency exchange forward contracts	0.0	0.0	0.0
Cash acquired with acquisition of Ariscom		0.0
Acquisition of subsidiaries, net of cash			0.0
Issuance of intercompany note, net			120.0
Purchases of fixed assets and other, net	0.0	0.0	0.0
Cash used in investing activities	0.0	0.0	120.0
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	0.0
Additional long-term borrowings			0.0
Borrowing under the intercompany note		0.0	(120.0)
Activity under stock incentive plans	0.0	0.0	0.0
Repurchase of Company's common shares	0.0	0.0	0.0
Payment of cash dividends to common shareholders	0.0	0.0	0.0
Cash (used in) provided by financing activities	0.0	0.0	(120.0)
Effect of exchange rate changes on cash	0.0	0.0	0.0
Change in cash	0.0	0.0	0.0
Cash, beginning of year	0.0	0.0	0.0
Cash, end of period	0.0	0.0	0.0
Other Subsidiaries and Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	20.6	46.5	(5.3)
Cash flows from investing activities:
Proceeds from sales of investments	554.8	532.1	920.4
Maturities and mandatory calls of fixed maturity investments	229.4	73.7	195.2
Purchases of investments	(680.2)	(640.8)	(1,165.2)
Change in short-term investments and foreign regulatory deposits	(142.2)	(10.8)	230.9
Settlements of foreign currency exchange forward contracts	(1.3)	(3.2)	4.4
Cash acquired with acquisition of Ariscom		15.6
Acquisition of subsidiaries, net of cash			130.1
Issuance of intercompany note, net			(120.0)
Purchases of fixed assets and other, net	(1.6)	(13.4)	(33.8)
Cash used in investing activities	(41.1)	(46.8)	162.0
Cash flows from financing activities:
Payment on the intercompany note	19.1
Payment on note payable	0.0
Additional long-term borrowings			0.0
Borrowing under the intercompany note		(19.0)	(60.0)
Activity under stock incentive plans	0.0	0.0	0.0
Repurchase of Company's common shares	0.0	0.0	0.0
Payment of cash dividends to common shareholders	0.0	0.0	0.0
Cash (used in) provided by financing activities	19.1	(19.0)	(60.0)
Effect of exchange rate changes on cash	(0.1)	(2.8)	(1.1)
Change in cash	(1.5)	(22.1)	95.6
Cash, beginning of year	105.8	127.9	32.3
Cash, end of period	104.3	105.8	127.9
Argo Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	38.4	72.4	20.8
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0	0.0
Purchases of investments	0.0	0.0	0.0
Change in short-term investments and foreign regulatory deposits	3.2	(3.4)	1.5
Settlements of foreign currency exchange forward contracts	(0.2)	(0.5)	0.9
Cash acquired with acquisition of Ariscom		0.0
Acquisition of subsidiaries, net of cash	0.0	0.0	(235.3)
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	0.0	(0.1)	(0.1)
Cash used in investing activities	3.0	(4.0)	(233.0)
Cash flows from financing activities:
Payment on the intercompany note	0.0
Payment on note payable	0.0
Additional long-term borrowings			125.0
Borrowing under the intercompany note		0.0	120.0
Activity under stock incentive plans	1.9	1.6	1.4
Repurchase of Company's common shares	0.0	(31.7)	(0.1)
Payment of cash dividends to common shareholders	(43.1)	(37.5)	(33.2)
Cash (used in) provided by financing activities	(41.2)	(67.6)	213.1
Effect of exchange rate changes on cash	0.0	0.0	0.0
Change in cash	0.2	0.8	0.9
Cash, beginning of year	1.7	0.9	0.0
Cash, end of period	1.9	1.7	0.9
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	124.3	182.4	149.5
Cash flows from investing activities:
Proceeds from sales of investments	1,297.3	1,067.7	809.5
Maturities and mandatory calls of fixed maturity investments	292.8	344.9	483.1
Purchases of investments	(1,303.8)	(1,508.3)	(1,495.6)
Change in short-term investments and foreign regulatory deposits	(351.4)	(105.0)	67.1
Settlements of foreign currency exchange forward contracts	1.8	2.2	(8.2)
Cash acquired with acquisition of Ariscom		0.0
Acquisition of subsidiaries, net of cash			0.0
Issuance of intercompany note, net			0.0
Purchases of fixed assets and other, net	(41.4)	(19.0)	(26.2)
Cash used in investing activities	(104.7)	(217.5)	(170.3)
Cash flows from financing activities:
Payment on the intercompany note	(19.1)
Payment on note payable	(0.6)
Additional long-term borrowings			0.0
Borrowing under the intercompany note		19.0	60.0
Activity under stock incentive plans	0.0	0.0	0.0
Repurchase of Company's common shares	0.0	0.0	(45.1)
Payment of cash dividends to common shareholders	0.0	0.0	0.0
Cash (used in) provided by financing activities	(19.7)	19.0	14.9
Effect of exchange rate changes on cash	0.0	0.0	0.0
Change in cash	(0.1)	(16.1)	(5.9)
Cash, beginning of year	31.7	47.8	53.7
Cash, end of period	$ 31.6	$ 31.7	$ 47.8