Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) (USD $) In Thousands	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Dec. 31, 2009
Reserve For Losses And Loss Expenses [Abstract]
Outstanding loss reserves	$ 1,404,996		$ 1,166,516
Reserves for losses incurred but not reported	3,840,557		3,712,672
Reserve for losses and loss expenses	$ 5,245,553	$ 5,251,304	$ 4,879,188	$ 4,889,825	$ 4,920,435	$ 4,761,772