Schedule III Supplemental Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	$ 168.0	$ 163.6	$ 160.2
Reserves for Losses and Loss Adjustment Expenses	5,595.0	5,406.0	5,157.6
UPR	1,466.8	1,464.8	1,410.9
Premium Revenue	1,910.1	1,780.5	1,729.7
Net investment income	187.6	112.7	151.1
Loss & LAE	1,314.6	1,208.8	1,220.7
Amortization (Deferral) DAC	(15.8)	(9.3)	12.4
Other Operating Expenses	718.1	677.0	653.6
Net Premiums Written	1,977.3	1,810.1	1,754.6
U.S. Operations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	103.7	98.2	89.7
Reserves for Losses and Loss Adjustment Expenses	3,422.4	3,091.9	2,775.1
UPR	973.7	939.2	896.1
Premium Revenue	1,283.7	1,207.6	1,119.9
Net investment income	119.4	80.3	100.0
Loss & LAE	908.2	768.7	690.4
Amortization (Deferral) DAC	(5.8)	(8.6)	(2.5)
Other Operating Expenses	425.1	398.3	371.2
Net Premiums Written	1,304.8	1,223.0	1,166.3
International Operations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	64.3	65.4	70.5
Reserves for Losses and Loss Adjustment Expenses	1,911.4	2,077.6	2,129.0
UPR	493.1	525.4	514.7
Premium Revenue	625.8	572.5	609.6
Net investment income	50.6	26.7	44.2
Loss & LAE	362.1	428.6	518.3
Amortization (Deferral) DAC	(10.0)	(0.7)	14.9
Other Operating Expenses	256.3	242.3	235.3
Net Premiums Written	671.7	586.6	588.1
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	261.2	236.5	253.5
UPR	0.0	0.2	0.1
Premium Revenue	0.6	0.4	0.2
Net investment income	3.6	4.0	5.7
Loss & LAE	44.3	11.5	12.0
Amortization (Deferral) DAC	0.0	0.0	0.0
Other Operating Expenses	1.0	1.7	2.4
Net Premiums Written	0.8	0.5	0.2
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	14.0	1.7	1.2
Loss & LAE	0.0	0.0	0.0
Amortization (Deferral) DAC	0.0	0.0	0.0
Other Operating Expenses	35.7	34.7	44.7
Net Premiums Written	$ 0.0	$ 0.0	$ 0.0