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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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			502.9	487.1
Prior accident years			19.8	(4.4)
Losses and LAE incurred during calendar year, net of reinsurance	$ 284.8	$ 245.5	522.7	482.7
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year			89.8	152.1
Prior accident years			432.4	327.0
Losses and LAE payments made during current calendar year, net of reinsurance:			522.2	479.1
Change in participation interest			(14.4)	(29.4)
Foreign exchange adjustments			(13.0)	(24.0)
Net reserves - end of period	2,536.0	2,487.6	2,536.0	2,487.6
Reinsurance recoverables on unpaid losses and LAE, end of period	2,199.7	1,755.3	2,199.7	1,755.3
Gross reserves - end of period	$ 4,735.7	$ 4,242.9	$ 4,735.7	$ 4,242.9	$ 4,654.6

[1] Derived from audited consolidated financial statements.