Reserves for Losses and Loss Adjustment Expenses - Reinsurance Recoverables (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Insurance [Abstract]
Net reserves beginning of the year	$ 2,110.9	$ 2,336.7
Net reserves from assumed retroactive insurance contract	0	13.0
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year	575.6	545.7
Prior accident years	(21.5)	(17.8)
Losses and LAE incurred during calendar year, net of reinsurance	554.1	527.9
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year	131.7	111.9
Prior accident years	429.0	450.1
Losses and LAE payments made during current calendar year, net of reinsurance:	560.7	562.0
Change in participation interest	10.4	(3.1)
Foreign exchange adjustments	(6.3)	5.4
Net reserves - end of period	2,108.4	2,317.9
Reinsurance recoverables on unpaid losses and LAE, end of period	1,114.5	910.0
Gross reserves - end of period	$ 3,222.9	$ 3,227.9