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, 2016	Dec. 31, 2015		Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 9,244,872	$ 8,970,641		$ 8,683,797
Net provision for losses and loss expenses:
Claims occuring during the current year	3,826,620	3,653,561		3,495,825	[1]
Decrease in estimates for claims occurring in prior years	(29,904)	(46,713)		(75,764)	[2]
Loss reserve discount accretion	49,084	49,422	[3]	70,506	[3]
Total	3,845,800	3,656,270		3,490,567
Net payments for claims:
Current year	1,052,452	914,637		898,944
Prior year	2,401,722	2,342,378		2,216,283
Total	3,454,174	3,257,015		3,115,227
Foreign currency translation	(46,233)	(125,024)		(88,496)
Net reserves at end of year	9,590,265	9,244,872		8,970,641
Ceded reserve at end of year	1,606,930	1,424,278		1,399,060
Gross reserves at end of year	11,197,195	10,669,150		10,369,701
Workers' Compensation Discount, Amount	18,929	20,357		21,306
Decrease in estimates for claims occurring in prior years, undiscounted basis	$ 59,175	$ 64,971		116,866
Undiscounted [Member]
Net payments for claims:
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years				30,000
Discounted [Member]
Net payments for claims:
Prior year				$ 12,000

[1] Claims occurring during the current year are net of loss reserve discounts of $18,929,000, $20,357,000 and $21,306,000 in 2016, 2015, and 2014 , respectively.

[2] The decrease in estimates for claims occurring in prior years is net of loss reserve discount. On an undiscounted basis, the estimates for claims occurring in prior years decreased by $59,175,000, $64,971,000 and $116,866,000 in 2016, 2015 and 2014, respectively.

[3] In 2014, the Company entered into a commutation agreement that resulted in a reduction in prior year workers' compensation reserves of $30 million on an undiscounted basis and $12 million on a discounted basis.