Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reserve For Losses And Loss Expenses [Abstract]
Gross liability at beginning of year	$ 4,879,188	$ 4,761,772	$ 4,576,828
Reinsurance recoverable at beginning of year	(927,588)	(919,991)	(888,314)
Net liability at beginning of year	3,951,600	3,841,781	3,688,514
Commutation of variable-rated reinsurance contracts	11,529	8,864
Current year	1,201,155	1,012,374	852,052
Prior years	(253,528)	(313,355)	(247,992)
Total incurred	959,156	707,883	604,060
Current year	142,162	98,646	42,320
Prior years	542,649	498,084	415,901
Total paid	684,811	596,730	458,221
Foreign exchange revaluation	(3,721)	(1,334)	7,428
Net liability at end of year	4,222,224	3,951,600	3,841,781
Reinsurance recoverable at end of year	1,002,919	927,588	919,991
Gross liability at end of year	$ 5,225,143	$ 4,879,188	$ 4,761,772