Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Net reserves for losses and loss adjustment expenses, beginning of year	$ 9,214,443	$ 8,964,945
Foreign currency movements	28,649	50,766
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	9,243,092	9,015,711
Incurred losses and loss adjustment expenses, current accident year	757,970	723,208
Incurred losses and loss adjustment expenses, prior accident years	(70,210)	(108,067)
Total incurred losses and loss adjustment expenses	687,760	615,141
Payments, current accident year	55,999	62,506
Payments, prior accident years	635,980	576,975
Total payments	691,979	639,481
Effect of foreign currency rate changes	(22)	(294)
Net reserves for losses and loss adjustment expenses, end of period	9,238,851	8,991,077
Reinsurance recoverable on unpaid losses	5,093,814	4,650,860
Gross reserves for losses and loss adjustment expenses, end of period	$ 14,332,665	$ 13,641,937