Segment Information - Schedule of Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net earned premiums	$ 162.3	$ 1,225.9	$ 1,089.8	$ 1,740.4
Net investment income	28.6	121.3	249.8	129.8
Net investment and other gains (losses)	(0.3)	(22.7)	20.0	(115.3)
Total revenue	190.6	1,324.5	1,359.6	1,754.9
Income (loss) before income taxes	2.2	(210.1)	(190.3)	(183.2)
Foreign currency exchange gains (losses)	(0.6)	(1.8)	1.0	5.0
Impairment of goodwill and intangibles	0.0	0.0	0.0	(28.5)
Non-operating expense	(13.1)	(41.1)	(17.6)	(51.5)
Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums	162.3		1,089.8
Net investment income	28.6		249.8
Total revenue	190.9		1,339.6
Income (loss) before income taxes		(122.0)		39.1
Corporate and Other
Segment Reporting Information [Line Items]
Net earned premiums	0.0	0.0	0.0	0.0
Net investment income	$ 0.0	0.0	$ 0.0	0.0
Income (loss) before income taxes		(22.5)		(32.0)
U.S. Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums		1,101.6		1,209.0
Net investment income		99.9		88.4
Income (loss) before income taxes		(127.5)		(22.9)
International Operations | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums		124.0		530.5
Net investment income		18.7		39.1
Income (loss) before income taxes		6.7		63.8
Run-off Lines | Operating Segments
Segment Reporting Information [Line Items]
Net earned premiums		0.3		0.9
Net investment income		2.7		2.3
Income (loss) before income taxes		$ (1.2)		$ (1.8)