Reserves For Losses And Loss Adjustment Expenses (Reinsurance Recoverables) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Net reserves beginning of the year	$ 2,336.7		$ 2,253.0
Net reserves from assumed retroactive insurance contract	13.0	[1]	0	[1]
Current accident year	545.7		662.8
Prior accident years	(17.8)		1.6
Losses and LAE incurred during calendar year, net of reinsurance	527.9		664.4
Current accident year	111.9		153.2
Prior accident years	450.1		417.1
Losses and LAE payments made during current calendar year, net of reinsurance:	562.0		570.3
Change in participation interest	(3.1)	[2]	31.2	[2]
Foreign exchange adjustments	5.4		12.0
Net reserves - end of period	2,317.9		2,390.3
Reinsurance recoverables on unpaid losses and LAE, end of period	910.0		959.3
Gross reserves - end of period	$ 3,227.9		$ 3,349.6

[1] Amount represents reserves assumed resulting from participation in Brazilian Motor Third-Party Liability Insurance Pool effective January 1, 2012.

[2] Amount represents (decrease) increase in reserves due to change in syndicate participation.