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, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 9,590,265	$ 9,244,872	$ 8,970,641
Net provision for losses and loss expenses:
Claims occuring during the current year	3,963,543	3,826,620	3,653,561
Decrease in estimates for claims occurring in prior years	(5,165)	(29,904)	(46,713)
Loss reserve discount accretion	43,970	49,084	49,422
Total	4,002,348	3,845,800	3,656,270
Net payments for claims:
Current year	1,027,405	1,052,452	914,637
Prior year	2,562,550	2,401,722	2,342,378
Total	3,589,955	3,454,174	3,257,015
Foreign currency translation	54,256	(46,233)	(125,024)
Net reserves at end of year	10,056,914	9,590,265	9,244,872
Ceded reserve at end of year	1,613,494	1,606,930	1,424,278
Gross reserves at end of year	11,670,408	11,197,195	10,669,150
Workers' Compensation Discount, Amount	22,064	18,929	20,357
Decrease in estimates for claims occurring in prior years, undiscounted basis	$ 32,132	$ 59,175	$ 64,971