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Losses and Loss Expense Reserve - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Beginning balance of net loss reserves, net of subrogation recoverable and reinsurance $ 6,230,780 $ 4,424,450
Current year:    
Establishment of new loss reserves, gross of subrogation and net of reinsurance 239,294 503,697
Claim payments, net of subrogation and reinsurance (11,508) (2,442)
Establishment of subrogation recoveries, net of reinsurance   (9,761)
Total current year 227,786 491,494
Prior year:    
Change in previously established loss reserves, gross of subrogation and net of reinsurance 487,037 1,683,831
Claim payments, net of subrogation and reinsurance (607,593) (179,440)
Change in previously established subrogation recoveries, net of reinsurance 15,861 (324,151)
Total prior year (104,695) 1,180,240
Changes in loss reserves 123,091 1,671,734
Net deconsolidation of certain VIEs   134,596
Ending loss reserves, net of subrogation recoverable and reinsurance $ 6,353,871 $ 6,230,780