Reserves for Losses and Loss Adjustment Expenses - Reinsurance Recoverables (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Abstract]
Net reserves beginning of the year	$ 2,137.1	$ 2,107.6
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	187.4	191.4
Prior accident years	(3.7)	(8.9)
Losses and LAE incurred during calendar year, net of reinsurance	183.7	182.5
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	19.8	27.9
Prior accident years	141.0	150.1
Losses and LAE payments made during current calendar year, net of reinsurance:	160.8	178.0
Change in participation interest	(1.2)	24.7
Foreign exchange adjustments	(6.0)	1.7
Net reserves - end of period	2,152.8	2,138.5
Reinsurance recoverables on unpaid losses and LAE, end of period	882.3	903.9
Gross reserves - end of period	$ 3,035.1	$ 3,042.4	$ 3,042.4	[1]

[1] Derived from audited consolidated financial statements.