Losses and Loss Expense Reserve - Summary of Balance of Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	$ 2,688,867	$ 2,720,266
Subrogation recoveries included in subrogation recoverable	785,519	829,469
Loss and loss expense reserves included in subrogation recoverable	$ 1,903,348	$ 1,890,797