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Losses and Loss Expense Reserve - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Unpaid Losses And Loss Expenses [Line Items]          
Net change in loss reserves     $ 123,091   $ 1,671,734
Total net loss expense reserves 138,023   138,023   86,171
Total loss and loss expenses (18,745) (60,238) 720,346 1,055,807  
Reinsurance recoveries of losses included in loss and loss expenses 5,502 1,892 27,354 26,634  
Loss reserves ceded to reinsurers 166,269   166,269   153,480
Subrogation recoveries 2,688,867   2,688,867   2,720,266
Subrogation recoveries, net of reinsurance $ 2,660,930   $ 2,660,930   $ 2,692,414