Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year	$ 2,906.1	$ 2,722.7	$ 2,562.9
Net AIL reserves acquired	0.0	27.9	0.0
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	1,176.3	1,201.1	1,082.6
Prior accident years	138.3	7.7	138.1
Losses and LAE incurred during calendar year, net of reinsurance	1,314.6	1,208.8	1,220.7
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	180.8	253.4	224.3
Prior accident years	688.4	866.4	806.0
Losses and LAE payments made during current calendar year, net of reinsurance:	869.2	1,119.8	1,030.3
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior	219.7	0.0	0.0
Change in participation interest	8.4	32.8	(14.4)
Foreign exchange adjustments	(17.0)	33.7	(16.2)
Net reserves - end of period	3,123.2	2,906.1	2,722.7
Reinsurance recoverables on unpaid losses and LAE, end of period	2,471.8	2,499.9	2,434.9
Reserves for losses and loss adjustment expenses	$ 5,595.0	$ 5,406.0	$ 5,157.6