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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	[1]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 2,562.9	$ 2,488.0
Net reserves acquired			0.0	49.4
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			799.9	764.3
Prior accident years			61.6	(4.1)
Losses and LAE incurred during calendar year, net of reinsurance	$ 338.8	$ 277.5	861.5	760.2
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			163.6	223.2
Prior accident years			615.8	469.0
Losses and LAE payments made during current calendar year, net of reinsurance:			779.4	692.2
Change in participation interest			(14.4)	(29.4)
Foreign exchange adjustments			(26.3)	(20.8)
Net reserves - end of period	2,604.3	2,555.2	2,604.3	2,555.2
Reinsurance recoverables on unpaid losses and LAE, end of period	2,238.2	1,736.5	2,238.2	1,736.5
Gross reserves - end of period	$ 4,842.5	$ 4,291.7	$ 4,842.5	$ 4,291.7	$ 4,654.6

[1] Derived from audited consolidated financial statements.