RESERVE FOR LOSSES AND LOSS EXPENSES (DETAILS 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable prior year reserve development	$ 78,431	$ 71,707	$ 179,686	$ 231,777
Current year net incurred losses			2,271,284	1,525,564
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	58.00%	44.00%	62.00%	52.00%
Professional Lines Insurance And Reinsurance Business [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Net favorable prior year reserve development	31,000	25,000	68,000	98,000
Earthquake [Member] | New Zealand [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses			396,000	85,000
Earthquake [Member] | Japan [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses			188,000
Earthquake [Member] | Chile [Member]
Additional Information Related to Net Incurred Losses [Line Items]
Current year net incurred losses				$ 130,000