Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 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	280.2	306.6
Prior accident years	3.4	1.0
Losses and LAE incurred during calendar year, net of reinsurance	283.6	307.6
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	19.0	33.0
Prior accident years	266.1	230.3
Losses and LAE payments made during current calendar year, net of reinsurance:	285.1	263.3
Divestitures	(31.0)	0.0
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior	0.0	207.8
Change in participation interest	48.0	19.8
Foreign exchange adjustments	(0.4)	(0.8)
Net reserves - end of period	3,138.3	2,761.6	$ 3,123.2
Reinsurance recoverables on unpaid losses and LAE, end of period	2,509.8	2,373.7
Gross reserves - end of period	$ 5,648.1	$ 5,135.3	$ 5,595.0