Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (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
Schedule of Investments [Line Items]
Gross realized investment gains	$ (0.6)	$ (1.8)	$ 1.0	$ 0.0
Foreign Currency Exchange Forward Contracts
Schedule of Investments [Line Items]
Gross realized investment gains	4.5	13.5	9.2	33.9
Gross realized investment losses	(3.6)	(12.4)	(5.3)	(47.2)
Net realized investment gains (losses) on foreign currency exchange forward contracts	0.9	1.1	3.9	(13.3)
Foreign Currency Exchange Forward Contracts | Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	4.5	12.0	2.8	30.0
Gross realized investment losses	(2.9)	(11.4)	(3.6)	(46.3)
Foreign Currency Exchange Forward Contracts | Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	0.0	1.5	6.4	3.9
Gross realized investment losses	$ (0.7)	$ (1.0)	$ (1.7)	$ (0.9)