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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	[1]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 2,722.7	$ 2,562.9
Net AIL reserves acquired			27.9	0.0
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			876.9	799.9
Prior accident years			6.1	61.6
Losses and LAE incurred during calendar year, net of reinsurance	$ 328.9	$ 338.8	883.0	861.5
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			188.2	163.6
Prior accident years			632.7	615.8
Losses and LAE payments made during current calendar year, net of reinsurance:			820.9	779.4
Change in participation interest			33.0	(14.4)
Foreign exchange adjustments			23.0	(26.3)
Net reserves - end of period	2,868.7	2,604.3	2,868.7	2,604.3
Reinsurance recoverables on unpaid losses and LAE, end of period	2,521.3	2,238.2	2,521.3	2,238.2
Gross reserves - end of period	$ 5,390.0	$ 4,842.5	$ 5,390.0	$ 4,842.5	$ 5,157.6

[1] Derived from audited consolidated financial statements.