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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 2,906.1	$ 2,722.7
Net AIL reserves acquired			0.0	27.9
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			884.9	876.9
Prior accident years			6.0	6.1
Losses and LAE incurred during calendar year, net of reinsurance	$ 311.7	$ 328.9	890.9	883.0
Net reserve ceded - reinsurance to close transaction for years of account 2017 and prior			219.7	0.0
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			100.5	188.2
Prior accident years			552.8	632.7
Losses and LAE payments made during current calendar year, net of reinsurance:			653.3	820.9
Change in participation interest			12.5	33.0
Foreign exchange adjustments			(7.3)	23.0
Net reserves - end of period	2,929.2	2,868.7	2,929.2	2,868.7	$ 2,906.1
Reinsurance recoverables on unpaid losses and LAE, end of period	2,510.3	2,521.3	2,510.3	2,521.3
Gross reserves - end of period	$ 5,439.5	$ 5,390.0	$ 5,439.5	$ 5,390.0	$ 5,406.0