Segment Information - Schedule of Revenue and Income (Loss) Before Income Taxes for Each Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net earned premiums	$ 234.3	$ 342.7	$ 837.9	$ 1,062.5
Net investment income	62.3	40.4	187.1	102.9
Net investment and other gains (losses)	(0.2)	(4.7)	30.8	(21.0)
Total revenue	296.4	378.4	1,055.8	1,144.4
Income (loss) before income taxes	(166.7)	(29.3)	(121.2)	(75.2)
Foreign currency exchange gains (losses)	3.8	3.3	3.1	(0.1)
Operating Segments
Segment Reporting Information [Line Items]
Income (loss) before income taxes	(173.1)	(16.6)	(149.5)	(14.9)
Corporate and Other
Segment Reporting Information [Line Items]
Income (loss) before income taxes	2.8	(11.3)	(5.6)	(39.2)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	210.4	314.1	761.9	950.9
Net investment income	52.3	33.2	158.4	84.7
Income (loss) before income taxes	(100.3)	(14.0)	(74.3)	(23.2)
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	23.9	28.5	75.8	111.4
Net investment income	7.6	6.2	22.8	15.8
Income (loss) before income taxes	(67.1)	(2.6)	(70.1)	9.3
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	0.0	0.1	0.2	0.2
Net investment income	2.4	1.0	5.9	2.4
Income (loss) before income taxes	$ (5.7)	$ 0.0	$ (5.1)	$ (1.0)