Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid 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
Reserve For Losses And Loss Expenses [Abstract]
Gross liability at beginning of year	$ 5,673,220	$ 5,331,418	$ 5,645,549	$ 5,225,143
Reinsurance recoverable at beginning of year	(1,163,503)	(1,056,780)	(1,141,110)	(1,002,919)
Net liability at beginning of year	4,509,717	4,274,638	4,504,439	4,222,224
Net losses incurred related to:
Current year	323,556	282,302	622,804	546,986
Prior years	(48,428)	(41,922)	(92,498)	(81,404)
Total incurred	275,128	240,380	530,306	465,582
Net paid losses related to:
Current year	21,003	18,226	24,584	19,840
Prior years	241,764	186,891	482,885	362,411
Total paid	262,767	205,117	507,469	382,251
Foreign exchange revaluation	(4,738)	(5,995)	(9,936)	(1,649)
Net liability at end of year	4,517,340	4,303,906	4,517,340	4,303,906
Reinsurance recoverable at end of year	1,179,525	1,073,612	1,179,525	1,073,612
Gross liability at end of year	$ 5,696,865	$ 5,377,518	$ 5,696,865	$ 5,377,518