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, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 15,661,820	$ 14,248,879	$ 12,848,362
Net provision for losses and loss expenses:
Claims occuring during the current year	7,083,999	6,311,780	5,774,713
Increase in estimates for claims occurring in prior years (2)	14,350	29,681	54,511
Loss reserve discount accretion	33,246	30,681	32,526
Total	7,131,595	6,372,142	5,861,750
Net payments for claims:
Current year	1,278,585	1,217,078	1,068,577
Prior year	4,205,845	3,764,532	3,279,333
Total	5,484,430	4,981,610	4,347,910
Foreign currency translation	(142,344)	22,409	(113,323)
Net reserves at end of year	17,166,641	15,661,820	14,248,879
Ceded reserve at end of year	3,201,389	3,077,832	2,762,344
Gross reserves at end of year	20,368,030	18,739,652	17,011,223
Increase in estimates for claims occurring in prior years (2)	(14,350)	(29,681)	(54,511)
Retrospective premium adjustments for claims occurring in prior years (3)	18,782	10,782	18,106
Net premium and reserve development on prior years	4,432	(18,899)	(36,405)
Workers' compensation discount, amount	49,000	47,000	35,000
Increase (decrease) in estimates for claims occurring in prior years, undiscounted basis	$ 13,000	$ (13,000)	$ 16,000