Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 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	$ (2,866)	$ (20,554)	(2,866)	(20,554)
Adjusted net reserves for losses and loss adjustment expenses, beginning of year			9,211,577	8,944,391
Incurred losses and loss adjustment expenses, current accident year	796,712	756,674	1,554,682	1,479,882
Incurred losses and loss adjustment expenses, prior accident years			(188,949)	(265,613)
Total incurred losses and loss adjustment expenses			1,365,733	1,214,269
Payments, current accident year			206,461	195,873
Payments, prior accident years			1,173,743	1,073,004
Total payments			1,380,204	1,268,877
Effect of foreign currency rate changes			760	(101)
Net reserves for losses and loss adjustment expenses, end of period	9,197,866	8,889,682	9,197,866	8,889,682
Reinsurance recoverable on unpaid losses	5,137,917	4,739,259	5,137,917	4,739,259
Gross reserves for losses and loss adjustment expenses, end of period	$ 14,335,783	$ 13,628,941	$ 14,335,783	$ 13,628,941