Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Net reserves for losses and loss adjustment expenses, beginning of year	$ 8,964,945	$ 8,108,717	$ 8,235,288
Foreign currency movements	(69,119)	110,079	(129,692)
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	8,895,826	8,218,796	8,105,596
Incurred losses and loss adjustment expenses, current accident year	3,371,699	3,367,223	2,555,902
Incurred losses and loss adjustment expenses, prior accident years	(551,040)	(497,627)	(493,495)
Total incurred losses and loss adjustment expenses	2,820,659	2,869,596	2,062,407
Payments, current accident year	666,515	671,112	532,140
Payments, prior accident years	1,835,027	1,513,580	1,529,206
Total payments	2,501,542	2,184,692	2,061,346
Effect of foreign currency rate changes	(500)	3,752	2,060
Net reserves for losses and loss adjustment expenses of acquired insurance companies	0	57,493	0
Net reserves for losses and loss adjustment expenses, end of year	9,214,443	8,964,945	8,108,717
Reinsurance recoverable on unpaid losses	5,062,036	4,619,336	2,006,945
Gross reserves for losses and loss adjustment expenses, end of year	$ 14,276,479	$ 13,584,281	$ 10,115,662