Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Earned premiums	$ 389.9	$ 480.6
Net investment income	29.7	37.7
Net investment and other gains (losses)	(17.9)	(34.5)
Total revenue	401.7	483.8
(Loss) income before income taxes	(42.3)	12.0
Foreign currency exchange gains (losses)	(2.7)	(2.9)
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	(2.9)	65.6
Corporate and Other
Segment Reporting Information [Line Items]
(Loss) income before income taxes	(18.8)	(16.2)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	325.6	336.4
Net investment income	24.4	25.6
(Loss) income before income taxes	(4.6)	43.9
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	64.3	144.2
Net investment income	4.6	11.4
(Loss) income before income taxes	2.4	22.7
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Net investment income	0.7	0.7
(Loss) income before income taxes	$ (0.7)	$ (1.0)