Reserves For Losses And Loss Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 8,411,851,000		$ 8,172,112,000		$ 7,999,521,000
Net provision for losses and loss expenses:
Claims occuring during the current year	3,221,393,000	[1]	2,997,995,000	[1]	2,791,860,000	[1]
Decrease in estimates for claims occurring in prior years	(78,810,000)	[2],[3]	(102,571,000)	[2],[3]	(181,282,000)	[2],[3]
Loss reserve discount accretion	54,441,000		53,055,000		47,787,000
Total	3,197,024,000		2,948,479,000		2,658,365,000
Net payments for claims:
Current year	822,787,000		698,834,000		765,440,000
Prior year	2,055,284,000		2,010,101,000		1,721,558,000
Total	2,878,071,000		2,708,935,000		2,486,998,000
Foreign currency translation	(47,007,000)		195,000		1,224,000
Net reserves at end of year	8,683,797,000		8,411,851,000		8,172,112,000
Ceded reserve at end of year	1,397,144,000		1,339,235,000		1,165,022,000
Gross reserves at end of year	10,080,941,000		9,751,086,000		9,337,134,000
Workers' Compensation Discount, Current Year Amount	22,680,000		26,078,000		43,286,000
Decrease in estimates for claims occurring in prior years, undiscounted basis	77,430,000		100,667,000		182,937,000
Favorable reserve development and return premiums	$ 98,000,000		$ 103,000,000		$ 182,000,000

[1] Claims occurring during the current year are net of loss reserve discounts of $22,680,000, $26,078,000 and $43,286,000 in 2013, 2012 and 2011, respectively.

[2] For certain retrospectively rated insurance polices and reinsurance agreements, reserve development is offset by additional or return premiums. Favorable reserve development, net of additional and return premiums, was $98 million, $103 million and $182 million in 2013, 2012 and 2011, respectively.

[3] 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 $77,430,000, $100,667,000 and $182,937,000 in 2013, 2012 and 2011, respectively.