Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Earned premiums	$ 329.9	$ 454.3	$ 719.8	$ 934.9
Net investment income	32.8	29.3	62.5	67.0
Net investment and other gains (losses)	1.6	(40.4)	(16.3)	(74.9)
Total revenue	364.3	443.2	766.0	927.0
(Loss) income before income taxes	(3.6)	(4.1)	(45.9)	7.9
Foreign currency exchange gains (losses)	(0.7)	10.3	(3.4)	7.4
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	4.6	44.1	1.7	109.7
Corporate and Other
Segment Reporting Information [Line Items]
(Loss) income before income taxes	(9.1)	(18.1)	(27.9)	(34.3)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	311.2	332.8	636.8	669.2
Net investment income	27.1	20.0	51.5	45.6
(Loss) income before income taxes	(4.6)	43.0	(9.2)	86.9
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	18.6	121.3	82.9	265.5
Net investment income	5.0	8.7	9.6	20.1
(Loss) income before income taxes	9.5	2.6	11.9	25.3
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.1	0.2	0.1	0.2
Net investment income	0.7	0.6	1.4	1.3
(Loss) income before income taxes	$ (0.3)	$ (1.5)	$ (1.0)	$ (2.5)