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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	[1]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year			$ 2,488.0	$ 2,180.2
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year			487.1	447.4
Prior accident years			(4.4)	5.7
Losses and LAE incurred during calendar year, net of reinsurance	$ 245.5	$ 230.6	482.7	453.1
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			152.1	88.9
Prior accident years			327.0	290.8
Losses and LAE payments made during current calendar year, net of reinsurance:			479.1	379.7
Change in participation interest			(29.4)	(23.2)
Foreign exchange adjustments			(24.0)	1.8
Net reserves - end of period	2,487.6	2,364.8	2,487.6	2,364.8
Reinsurance recoverables on unpaid losses and LAE, end of period	1,755.3	1,268.0	1,755.3	1,268.0
Gross reserves - end of period	$ 4,242.9	$ 3,632.8	4,242.9	3,632.8	$ 4,201.0
Maybrooke [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves acquired			0.0	132.6
Ariscom [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves acquired			$ 49.4	$ 0.0

[1] Derived from audited consolidated financial statements.