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, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 8,108,717
Effect of foreign currency rate changes on beginning of year balance	18,857	(69,119)	110,079
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	9,233,300	8,895,826	8,218,796
Incurred losses and loss adjustment expenses, current accident year	3,426,441	3,371,699	3,367,223
Incurred losses and loss adjustment expenses, prior accident years	(535,307)	(551,040)	(497,627)
Total incurred losses and loss adjustment expenses	2,891,134	2,820,659	2,869,596
Payments, current accident year	671,208	666,515	671,112
Payments, prior accident years	1,979,032	1,835,027	1,513,580
Total payments	2,650,240	2,501,542	2,184,692
Effect of foreign currency rate changes on current year activity	1,067	(500)	3,752
Net reserves for losses and loss adjustment expenses of acquired insurance companies	0	0	57,493
Net reserves for losses and loss adjustment expenses, end of year	9,475,261	9,214,443	8,964,945
Reinsurance recoverable on unpaid losses	5,253,415	5,062,036	4,619,336
Gross reserves for losses and loss adjustment expenses, end of year	$ 14,728,676	$ 14,276,479	$ 13,584,281