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, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 12,848,362	$ 11,620,393	$ 10,697,998
Net provision for losses and loss expenses:
Claims occuring during the current year	5,774,713	4,921,191	4,432,937
Increase in estimates for claims occurring in prior years (3)	54,511	863	627
Loss reserve discount accretion	32,526	31,906	35,142
Total	5,861,750	4,953,960	4,468,706
Net payments for claims:
Current year	1,068,577	887,896	921,054
Prior year	3,279,333	2,777,798	2,677,595
Total	4,347,910	3,665,694	3,598,649
Foreign currency translation	(113,323)	(60,297)	46,411
Net reserves at end of year	14,248,879	12,848,362	11,620,393
Ceded reserve at end of year	2,762,344	2,542,526	2,164,037
Gross reserves at end of year	17,011,223	15,390,888	13,784,430
Increase in estimates for claims occurring in prior years (3)	(54,511)	(863)	(627)
Retrospective premium adjustments for claims occurring in prior years (4)	18,106	7,510	16,807
Net premium and reserve development on prior years	(36,405)	6,647	16,180
Workers' compensation discount, amount	35,000	21,000	10,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	16,000	(19,000)	(21,000)
Cumulative Effect, Period Of Adoption, Adjustment
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	0	0	5,927
Net payments for claims:
Net reserves at end of year		0	0
Cumulative Effect, Period Of Adoption, Adjusted Balance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 12,848,362	11,620,393	10,703,925
Net payments for claims:
Net reserves at end of year		$ 12,848,362	$ 11,620,393