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, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 11,620,393	$ 10,697,998	$ 10,248,883
Net provision for losses and loss expenses:
Claims occuring during the current year	4,921,191	4,432,937	4,057,989
Increase in estimates for claims occurring in prior years (3)	863	627	34,079
Loss reserve discount accretion	31,906	35,142	39,048
Total	4,953,960	4,468,706	4,131,116
Net payments for claims:
Current year	887,896	921,054	985,599
Prior year	2,777,798	2,677,595	2,673,803
Total	3,665,694	3,598,649	3,659,402
Foreign currency translation	(60,297)	46,411	(22,599)
Net reserves at end of year	12,848,362	11,620,393	10,697,998
Ceded reserve at end of year	2,542,526	2,164,037	1,885,251
Gross reserves at end of year	15,390,888	13,784,430	12,583,249
Increase in estimates for claims occurring in prior years (3)	(863)	(627)	(34,079)
Retrospective premium adjustments for claims occurring in prior years (4)	7,510	16,807	53,511
Net favorable premium and reserve development on prior years	6,647	16,180	19,432
Workers' compensation discount, amount	21,000	10,000	20,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	19,000	(21,000)	(19,000)
Cumulative Effect, Period Of Adoption, Adjustment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	0	5,927	0
Net payments for claims:
Net reserves at end of year		0	5,927
Cumulative Effect, Period Of Adoption, Adjusted Balance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 11,620,393	10,703,925	10,248,883
Net payments for claims:
Net reserves at end of year		$ 11,620,393	$ 10,703,925