Reserves for Losses and Loss Adjustment Expenses -Schedule of Reserves for Losses and Loss Adjustment Expenses (Detail) $ in Millions	1 Months Ended	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Nov. 15, 2023 USD ($) agreement	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year	$ 2,734.6	$ 2,734.6	$ 2,213.1	$ 2,747.1	$ 2,213.1	$ 3,123.2
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	94.2	94.2	775.3	763.4		1,102.2
Prior accident years	0.0	0.0	267.9	254.9		64.7
Losses and LAE incurred during calendar year, net of reinsurance	94.2	94.2	1,043.2	1,018.3		1,166.9
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	33.5		100.6	91.4		171.5
Prior accident years	51.8		427.3	687.0		828.0
Losses and LAE payments made during current calendar year, net of reinsurance:	85.3		527.9	778.4		999.5
Add/(Deduct):
Divestitures	0.0		24.4	0.0		(35.2)
Reclassified to liabilities held-for-sale		0.0	0.0		0.0	(313.0)
Net reserves ceded to Syndicate 1200		0.0	0.0		0.0	(129.6)
Deferred gain on U.S. loss portfolio transfer, net of amortization	0.0		(40.9)	0.0		0.0
Loss portfolio transfer:
Retroactive reinsurance	0.0		21.7	0.0		0.0
Change in participation interest	0.0		0.0	0.0		34.3
Purchase accounting adjustment	0.0		0.0	19.4		0.0
Total deductions	0.0		5.2	19.4		(1,060.1)
Foreign exchange adjustments	3.6		1.0	(2.5)		(17.4)
Net reserves - end of period	2,747.1	2,747.1	2,734.6	3,003.9	2,747.1	2,213.1
Reinsurance recoverables on unpaid losses and LAE, end of year	2,797.4	2,797.4	2,791.8	2,794.7	2,797.4	2,838.5
Reserves for losses and loss adjustment expenses	$ 5,544.5	5,544.5	$ 5,526.4	$ 5,798.6	5,544.5	5,051.6
Number of retroactive reinsurance agreements entered into | agreement			2
Net reserves ceded to Syndicate 1200		0.0	$ 0.0		0.0	129.6
United States (Federal)
Loss portfolio transfer:
Purchase accounting adjustment		0.0	0.0		0.0	(472.6)
Syndicate 1200
Loss portfolio transfer:
Purchase accounting adjustment		$ 0.0	$ 0.0		$ 0.0	$ (144.0)