Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	278.2	240.4
Prior accident years	2.7	(2.5)
Losses and LAE incurred during calendar year, net of reinsurance	280.9	237.9
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	72.6	34.7
Prior accident years	263.8	221.0
Losses and LAE payments made during current calendar year, net of reinsurance:	336.4	255.7
Change in participation interest	32.5	(14.6)
Foreign exchange adjustments	(25.6)	(9.1)
Net reserves - end of period	2,674.1	2,521.4
Reinsurance recoverables on unpaid losses and LAE, end of period	2,393.5	2,147.5
Gross reserves - end of period	$ 5,067.6	$ 4,668.9	$ 5,157.6

[1] Derived from audited consolidated financial statements.