Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Earned premiums	$ 454.3	$ 470.3	$ 934.9	$ 936.4
Net investment income	29.3	52.7	67.0	97.1
Net realized investment gains (losses)	(40.4)	24.7	(74.9)	37.8
Total revenue	443.2	547.7	927.0	1,071.3
(Loss) income before income taxes	(4.1)	78.7	7.9	109.9
Foreign currency exchange gains (losses)	10.3	(4.4)	7.4	(5.7)
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	44.1	73.0	109.7	98.4
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	0.0	4.0	0.0	6.8
(Loss) income before income taxes	(18.1)	(14.6)	(34.3)	(20.6)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	332.8	314.5	669.2	628.9
Net investment income	20.0	33.7	45.6	62.5
(Loss) income before income taxes	43.0	53.6	86.9	90.2
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	121.3	155.7	265.5	307.2
Net investment income	8.7	13.9	20.1	25.9
(Loss) income before income taxes	2.6	19.5	25.3	7.7
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.2	0.1	0.2	0.3
Net investment income	0.6	1.1	1.3	1.9
(Loss) income before income taxes	$ (1.5)	$ (0.1)	$ (2.5)	$ 0.5