Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) $ in Millions	3 Months Ended		11 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Nov. 15, 2023 USD ($) agreement	Dec. 31, 2023 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves - beginning of the year	$ 2,747.1	$ 2,213.1	$ 2,213.1
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	195.2	236.0
Prior accident years	24.8	48.6
Losses and LAE incurred during calendar year, net of reinsurance	220.0	284.6
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	6.7	36.8
Prior accident years	174.8	149.1
Losses and LAE payments made during current calendar year, net of reinsurance:	181.5	185.9
Divestitures	0.0	24.4
Retroactive reinsurance	0.0	21.7
Total net reserve adjustments	0.0	46.1
Foreign exchange adjustments	(1.3)	2.1
Net reserves - end of period	2,784.3	2,360.0		$ 2,747.1
Reinsurance recoverables on unpaid losses and LAE, end of period	2,758.8	2,824.1
Gross reserves - end of period	$ 5,543.1	$ 5,184.1		$ 5,544.5
Number of retroactive reinsurance agreements entered into | agreement			2