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, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 14,248,879	$ 12,848,362	$ 11,620,393
Net provision for losses and loss expenses:
Claims occuring during the current year	6,311,780	5,774,713	4,921,191
Increase in estimates for claims occurring in prior years (2)	29,681	54,511	863
Loss reserve discount accretion	30,681	32,526	31,906
Total	6,372,142	5,861,750	4,953,960
Net payments for claims:
Current year	1,217,078	1,068,577	887,896
Prior year	3,764,532	3,279,333	2,777,798
Total	4,981,610	4,347,910	3,665,694
Foreign currency translation	22,409	(113,323)	(60,297)
Net reserves at end of year	15,661,820	14,248,879	12,848,362
Ceded reserve at end of year	3,077,832	2,762,344	2,542,526
Gross reserves at end of year	18,739,652	17,011,223	15,390,888
Increase in estimates for claims occurring in prior years (2)	(29,681)	(54,511)	(863)
Retrospective premium adjustments for claims occurring in prior years (3)	10,782	18,106	7,510
Net premium and reserve development on prior years	(18,899)	(36,405)	6,647
Workers' compensation discount, amount	47,000	35,000	21,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	$ 13,000	$ (16,000)	$ (19,000)