Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Net earned premiums	$ 313.7	$ 389.9
Net investment income	58.8	29.7
Net investment and other gains (losses)	6.2	(17.9)
Total revenue	378.7	401.7
Pre-Tax Income (Loss)	27.2	(42.3)
Foreign currency exchange gains (losses)	2.0	(2.7)
Operating Segments
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)	24.2	(2.9)
Corporate and Other
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)	(5.2)	(18.8)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	286.7	325.6
Net investment income	50.0	24.4
Pre-Tax Income (Loss)	32.2	(4.6)
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	26.9	64.3
Net investment income	7.2	4.6
Pre-Tax Income (Loss)	(7.8)	2.4
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	0.1	0.0
Net investment income	1.6	0.7
Pre-Tax Income (Loss)	$ (0.2)	$ (0.7)