Schedule III Supplemental Insurance Information (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	$ 160.2				$ 167.3				$ 160.2	$ 167.3	$ 160.4
Reserves for Losses and Loss Adjustment Expenses	5,157.6				4,654.6				5,157.6	4,654.6	4,201.0
UPR	1,410.9				1,300.9				1,410.9	1,300.9	1,207.7
Premium Revenue	425.8	$ 451.5	$ 431.7	$ 420.5	452.4	$ 446.9	$ 417.7	$ 414.7	1,729.5	1,731.7	1,572.3
Net investment income									151.1	133.1	140.0
Loss & LAE									1,220.7	1,040.8	1,050.2
Net deferral of policy acquisition costs									12.4	(9.9)	(11.9)
Other Operating Expenses	678.2				664.6				678.2	664.6	647.3
Net Premiums Written									1,754.4	1,765.5	1,653.5
U.S. Operations [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	89.7				87.2				89.7	87.2	80.8
Reserves for Losses and Loss Adjustment Expenses	2,775.1				2,498.9				2,775.1	2,498.9	2,196.1
UPR	896.1				793.3				896.1	793.3	695.1
Premium Revenue									1,119.7	1,078.9	936.6
Net investment income									100.0	82.9	87.2
Loss & LAE									690.4	628.2	528.1
Net deferral of policy acquisition costs									(2.5)	(6.4)	(17.3)
Other Operating Expenses	366.2				361.2				366.2	361.2	336.4
Net Premiums Written									1,166.1	1,125.7	1,031.8
International Operations [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	70.5				80.1				70.5	80.1	79.6
Reserves for Losses and Loss Adjustment Expenses	2,129.0				1,890.1				2,129.0	1,890.1	1,723.0
UPR	514.7				507.6				514.7	507.6	512.6
Premium Revenue									609.6	652.5	635.8
Net investment income									44.2	32.9	32.7
Loss & LAE									518.3	400.3	504.8
Net deferral of policy acquisition costs									14.9	(3.5)	5.4
Other Operating Expenses	265.1				249.3				265.1	249.3	236.8
Net Premiums Written									588.1	639.5	621.7
Run Off Lines [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	0.0				0.0				0.0	0.0	0.0
Reserves for Losses and Loss Adjustment Expenses	253.5				265.6				253.5	265.6	281.9
UPR	0.1				0.0				0.1	0.0	0.0
Premium Revenue									0.2	0.3	(0.1)
Net investment income									5.7	8.1	9.3
Loss & LAE									12.0	12.3	17.3
Net deferral of policy acquisition costs									0.0	0.0	0.0
Other Operating Expenses	2.4				3.9				2.4	3.9	8.3
Net Premiums Written									0.2	0.3	0.0
Corporate and Other [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
DAC	0.0				0.0				0.0	0.0	0.0
Reserves for Losses and Loss Adjustment Expenses	0.0				0.0				0.0	0.0	0.0
UPR	0.0				0.0				0.0	0.0	0.0
Premium Revenue									0.0	0.0	0.0
Net investment income									1.2	9.2	10.8
Loss & LAE									0.0	0.0	0.0
Net deferral of policy acquisition costs									0.0	0.0	0.0
Other Operating Expenses	$ 44.5				$ 50.2				44.5	50.2	65.8
Net Premiums Written									$ 0.0	$ 0.0	$ 0.0