Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Earned premiums	$ 455.0	$ 487.5	$ 1,389.9	$ 1,423.9
Net investment income	34.0	46.1	100.9	143.2
Net investment and other gains (losses)	(44.7)	(5.3)	(119.6)	32.5
Total revenue	444.3	528.3	1,371.2	1,599.6
(Loss) income before income taxes	(52.8)	28.2	(44.9)	138.1
Foreign currency exchange gains (losses)	9.1	1.3	16.5	(4.4)
Impairment of goodwill and intangible assets	(28.5)	0.0	(28.5)	0.0
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	30.6	43.2	140.2	140.2
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	0.0	3.7	0.0	10.5
(Loss) income before income taxes	(19.3)	(11.0)	(53.5)	(30.2)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	329.3	323.5	998.5	952.4
Net investment income	23.1	29.2	68.7	91.7
(Loss) income before income taxes	26.6	39.9	113.5	130.1
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	125.2	163.9	390.7	471.1
Net investment income	10.3	12.3	30.4	38.2
(Loss) income before income taxes	3.3	6.9	28.6	14.6
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.5	0.1	0.7	0.4
Net investment income	0.6	0.9	1.8	2.8
(Loss) income before income taxes	$ 0.7	$ (3.6)	$ (1.9)	$ (4.5)