Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net earned premiums	$ 342.7	$ 455.0	$ 1,062.5	$ 1,389.9
Net investment income	40.4	34.0	102.9	100.9
Net investment and other gains (losses)	(4.7)	(44.7)	(21.0)	(119.6)
Total revenue	378.4	444.3	1,144.4	1,371.2
Pre-Tax Income (Loss)	(29.3)	(52.8)	(75.2)	(44.9)
Foreign currency exchange gains (losses)	3.3	9.1	(0.1)	16.5
Impairment of goodwill and intangible assets	0.0	(28.5)	0.0	(28.5)
Operating Segments
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)	(16.6)	30.6	(14.9)	140.2
Corporate and Other
Segment Reporting Information [Line Items]
Pre-Tax Income (Loss)	(11.3)	(19.3)	(39.2)	(53.5)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	314.1	329.3	950.9	998.5
Net investment income	33.2	23.1	84.7	68.7
Pre-Tax Income (Loss)	(14.0)	26.6	(23.2)	113.5
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	28.5	125.2	111.4	390.7
Net investment income	6.2	10.3	15.8	30.4
Pre-Tax Income (Loss)	(2.6)	3.3	9.3	28.6
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	0.1	0.5	0.2	0.7
Net investment income	1.0	0.6	2.4	1.8
Pre-Tax Income (Loss)	$ 0.0	$ 0.7	$ (1.0)	$ (1.9)