Reserves For Losses And Loss Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 10,248,883	$ 10,056,914	$ 9,590,265
Net provision for losses and loss expenses:
Claims occuring during the current year	4,057,989	3,926,489	3,963,543
Increase (decrease) in estimates for claims occurring in prior years (2)	34,079	6,831	(5,165)
Loss reserve discount accretion	39,048	41,382	43,970
Total	4,131,116	3,974,702	4,002,348
Net payments for claims:
Current year	985,599	964,808	1,027,405
Prior year	2,673,803	2,700,077	2,562,550
Total	3,659,402	3,664,885	3,589,955
Foreign currency translation	(22,599)	(117,848)	54,256
Net reserves at end of year	10,697,998	10,248,883	10,056,914
Ceded reserve at end of year	1,885,251	1,717,565	1,613,494
Gross reserves at end of year	12,583,249	11,966,448	11,670,408
(Increase) decrease in estimates for claims occurring in prior years (2)	(34,079)	(6,831)	5,165
Retrospective premium adjustments for claims occurring in prior years (3)	53,511	45,638	32,162
Net favorable premium and reserve development on prior years	19,432	38,807	37,327
Workers' compensation discount, amount	20,000	24,000	22,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	$ 19,000	$ (4,000)	$ (32,000)