Segment Information - Schedule of Earned Premiums by Geographic Location (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 162.3	$ 1,225.9	$ 1,089.8	$ 1,740.4
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	147.4	1,101.9	989.0	1,209.9
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.0	48.3	0.0	480.2
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	14.9	75.7	100.8	38.0
Malta
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.0	0.0	0.0	3.9
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 0.0	$ 0.0	$ 0.0	$ 8.4