Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) agreement	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves - beginning of the year	$ 2,213.1	$ 3,123.2
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	236.0	280.2
Prior accident years	48.6	3.4
Losses and LAE incurred during calendar year, net of reinsurance	284.6	283.6
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	36.8	19.0
Prior accident years	149.1	266.1
Losses and LAE payments made during current calendar year, net of reinsurance:	185.9	285.1
Divestitures	24.4	(31.0)
Retroactive reinsurance	21.7	0.0
Change in participation interest	0.0	48.0
Total net reserve adjustments	46.1	17.0
Foreign exchange adjustments	2.1	(0.4)
Net reserves - end of period	2,360.0	3,138.3	$ 2,213.1
Reinsurance recoverables on unpaid losses and LAE, end of period	2,824.1	2,509.8
Gross reserves - end of period	$ 5,184.1	$ 5,648.1	$ 5,051.6
Number of retroactive reinsurance agreements | agreement	2