Reserves for Losses and Loss Adjustment Expenses - Schedule of Reserves for Losses and Loss Adjustment Expenses (Details) $ in Millions	3 Months Ended		9 Months Ended		11 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	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			583.8	680.9
Prior accident years			209.7	126.0
Losses and LAE incurred during calendar year, net of reinsurance	$ 346.8	$ 280.9	793.5	806.9
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			53.2	94.9
Prior accident years			552.6	361.6
Losses and LAE payments made during current calendar year, net of reinsurance:			605.8	456.5
Add/(Deduct):
Divestitures			0.0	24.4
Retroactive reinsurance			0.0	21.7
Deferred gain on U.S. loss portfolio transfer, net of amortization			0.0	(16.8)
Purchase accounting adjustment			19.4	0.0
Total net reserve adjustments			19.4	29.3
Foreign exchange adjustments			(1.6)	0.9
Net reserves - end of period	2,952.6	2,593.7	2,952.6	2,593.7
Reinsurance recoverables on unpaid losses and LAE, end of period	2,840.3	2,735.0	2,840.3	2,735.0
Gross reserves - end of period	$ 5,792.9	$ 5,328.7	$ 5,792.9	$ 5,328.7		$ 5,544.5
Number of retroactive reinsurance agreements entered into | agreement					2