Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 3,123.2	$ 2,906.1
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			826.8	884.9
Prior accident years			31.6	6.0
Losses and LAE incurred during calendar year, net of reinsurance	$ 298.8	$ 311.7	858.4	890.9
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			112.0	100.5
Prior accident years			623.2	552.8
Losses and LAE payments made during current calendar year, net of reinsurance:			735.2	653.3
Divestitures			(35.2)	0.0
Loss portfolio transfer (for years of account 2019 and 2018)			(175.5)	0.0
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior			0.0	219.7
Change in participation interest			32.2	12.5
Foreign exchange adjustments			(10.6)	(7.3)
Net reserves - end of period	3,057.3	2,929.2	3,057.3	2,929.2	$ 3,123.2
Reinsurance recoverables on unpaid losses and LAE, end of period	2,674.1	2,510.3	2,674.1	2,510.3
Gross reserves - end of period	$ 5,731.4	$ 5,439.5	$ 5,731.4	$ 5,439.5	$ 5,595.0