Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Earned premiums	$ 480.6	$ 466.1
Net investment income	37.7	44.4
Net realized investment and other gains (losses)	(34.5)	13.1
Total revenue	483.8	523.6
(Loss) income before income taxes	12.0	31.2
Foreign currency exchange losses	(2.9)	(1.3)
Operating Segments
Segment Reporting Information [Line Items]
(Loss) income before income taxes	65.6	25.4
Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	0.0	2.8
(Loss) income before income taxes	(16.2)	(6.0)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	336.4	314.4
Net investment income	25.6	28.8
(Loss) income before income taxes	43.9	36.6
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	144.2	151.5
Net investment income	11.4	12.0
(Loss) income before income taxes	22.7	(11.8)
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Earned premiums	0.0	0.2
Net investment income	0.7	0.8
(Loss) income before income taxes	$ (1.0)	$ 0.6