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, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 10,697,998	$ 10,248,883	$ 10,056,914
Net provision for losses and loss expenses:
Claims occuring during the current year	4,432,937	4,057,989	3,926,489
Increase in estimates for claims occurring in prior years (3)	627	34,079	6,831
Loss reserve discount accretion	35,142	39,048	41,382
Total	4,468,706	4,131,116	3,974,702
Net payments for claims:
Current year	921,054	985,599	964,808
Prior year	2,677,595	2,673,803	2,700,077
Total	3,598,649	3,659,402	3,664,885
Foreign currency translation	46,411	(22,599)	(117,848)
Net reserves at end of year	11,620,393	10,697,998	10,248,883
Ceded reserve at end of year	2,164,037	1,885,251	1,717,565
Gross reserves at end of year	13,784,430	12,583,249	11,966,448
Increase in estimates for claims occurring in prior years (3)	(627)	(34,079)	(6,831)
Retrospective premium adjustments for claims occurring in prior years (4)	16,807	53,511	45,638
Net favorable premium and reserve development on prior years	16,180	19,432	38,807
Workers' compensation discount, amount	10,000	20,000	24,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	21,000	(19,000)	(4,000)
Cumulative Effect, Period Of Adoption, Adjustment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	5,927	0	0
Net payments for claims:
Net reserves at end of year		5,927	0
Cumulative Effect, Period Of Adoption, Adjusted Balance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 10,703,925	10,248,883	10,056,914
Net payments for claims:
Net reserves at end of year		$ 10,703,925	$ 10,248,883