Schedule III Supplemental Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	$ 107.0	$ 168.0	$ 163.6
Reserves for Losses and Loss Adjustment Expenses	5,051.6	5,595.0	5,406.0
UPR	1,039.9	1,466.8	1,464.8
Premium Revenue	1,740.4	1,910.1	1,780.5
Net investment income	129.8	187.6	112.7
Loss & LAE	1,166.9	1,314.6	1,208.8
Amortization (Deferral) DAC	(10.5)	(15.8)	(9.3)
Other Operating Expenses	681.2	718.1	677.0
Net Premiums Written	1,741.5	1,977.3	1,810.1
U.S. Operations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	109.4	103.7	98.2
Reserves for Losses and Loss Adjustment Expenses	3,718.1	3,422.4	3,091.9
UPR	893.4	973.7	939.2
Premium Revenue	1,209.0	1,283.7	1,207.6
Net investment income	88.4	119.4	80.3
Loss & LAE	870.1	908.2	768.7
Amortization (Deferral) DAC	(3.5)	(5.8)	(8.6)
Other Operating Expenses	436.3	425.1	398.3
Net Premiums Written	1,196.2	1,304.8	1,223.0
International Operations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	(2.4)	64.3	65.4
Reserves for Losses and Loss Adjustment Expenses	1,100.4	1,911.4	2,077.6
UPR	146.5	493.1	525.4
Premium Revenue	530.5	625.8	572.5
Net investment income	39.1	50.6	26.7
Loss & LAE	293.9	362.1	428.6
Amortization (Deferral) DAC	(7.0)	(10.0)	(0.7)
Other Operating Expenses	212.3	256.3	242.3
Net Premiums Written	544.5	671.7	586.6
International Operations | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Underwriting Agency Limited
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Gross reserves, net of intercompany gross ups	993.4
Run-off Lines
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	0.0	0.0	0.0
Reserves for Losses and Loss Adjustment Expenses	233.1	261.2	236.5
UPR	0.0	0.0	0.2
Premium Revenue	0.9	0.6	0.4
Net investment income	2.3	3.6	4.0
Loss & LAE	2.9	44.3	11.5
Amortization (Deferral) DAC	0.0	0.0	0.0
Other Operating Expenses	1.6	1.0	1.7
Net Premiums Written	0.8	0.8	0.5
Corporate and Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	0.0	0.0	0.0
Reserves for Losses and Loss Adjustment Expenses	0.0	0.0	0.0
UPR	0.0	0.0	0.0
Premium Revenue	0.0	0.0	0.0
Net investment income	0.0	14.0	1.7
Loss & LAE	0.0	0.0	0.0
Amortization (Deferral) DAC	0.0	0.0	0.0
Other Operating Expenses	31.0	35.7	34.7
Net Premiums Written	$ 0.0	$ 0.0	$ 0.0