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RESERVE FOR LOSSES AND LOSS EXPENSES (DETAILS) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses        
Gross reserve for losses and loss expenses, beginning of period     $ 9,058,731 $ 8,425,045
Less reinsurance recoverable on unpaid losses, beginning of period     (1,825,617) (1,736,823)
Net reserve for unpaid losses and loss expenses, beginning of period     7,233,114 6,688,222
Net incurred losses and loss expenses related to:        
Current year     1,177,926 1,098,168
Prior years (42,115) (74,612) (96,613) (119,840)
Net incurred losses and loss expenses     1,081,313 978,328
Net paid losses and loss expenses related to:        
Current year     (54,732) (90,041)
Prior years     (732,135) (731,363)
Net paid losses and loss expenses     (786,867) (821,404)
Foreign exchange and other     (130,536) (10,129)
Net reserve for unpaid losses and loss expenses, end of period 7,397,024 6,835,017 7,397,024 6,835,017
Reinsurance recoverable on unpaid losses, end of period 1,945,793 1,765,634 1,945,793 1,765,634
Gross reserve for losses and loss expenses, end of period $ 9,342,817 $ 8,600,651 $ 9,342,817 $ 8,600,651