Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 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			539.9	579.4
Prior accident years			19.7	(0.2)
Losses and LAE incurred during calendar year, net of reinsurance	$ 276.0	$ 271.6	559.6	579.2
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			52.2	59.7
Prior accident years			454.7	376.0
Losses and LAE payments made during current calendar year, net of reinsurance:			506.9	435.7
Divestitures			(35.2)	0.0
Loss portfolio transfer (for years of account 2019 and 2018)			(181.2)	0.0
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior			0.0	(207.8)
Change in participation interest			28.6	12.8
Foreign exchange adjustments			1.2	6.1
Net reserves - end of period	2,989.3	2,860.7	2,989.3	2,860.7	$ 3,123.2
Reinsurance recoverables on unpaid losses and LAE, end of period	2,606.2	2,377.9	2,606.2	2,377.9
Gross reserves - end of period	$ 5,595.5	$ 5,238.6	$ 5,595.5	$ 5,238.6	$ 5,595.0