RESERVE FOR LOSSES AND LOSS EXPENSES (DETAILS 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable prior year reserve development	$ 257,461	$ 313,055	$ 423,172
Current year net incurred losses	2,932,513	1,990,187	1,847,044
Short Tail Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Percentage of net favorable prior year reserve development	69.00%	58.00%	65.00%
Professional Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable prior year reserve development	105,000	117,000	143,000
Earthquake [Member] \| New Zealand [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses	425,000	138,000
Earthquake [Member] \| Japan [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses	222,000
Flood [Member] \| Thailand [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses	$ 66,000