Reserves For Losses And Loss Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Insurance Loss Reserves [Abstract]
Workers' Compensation Discount, Current Year Amount	$ 26,078,000		$ 43,286,000		$ 67,763,000
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves	8,172,112,000		7,999,521,000		8,147,782,000
Net provision for losses and loss expenses:
Claim occurring during the current year	2,997,995,000	[1]	2,791,860,000	[1]	2,509,933,000	[1]
Decrease in estimates for claims occurring in prior years	(102,571,000)	[2],[3]	(181,282,000)	[2],[3]	(253,248,000)	[2],[3]
Loss reserve discount accretion	53,055,000		47,787,000		53,182,000
Total net incurred	2,948,479,000		2,658,365,000		2,309,867,000
Net payments for claims:
Current year	698,834,000		765,440,000		641,570,000
Prior year	2,010,101,000		1,721,558,000		1,811,507,000
Total net paid	2,708,935,000		2,486,998,000		2,453,077,000
Foreign currency translation	195,000		1,224,000		(5,051,000)
Net reserves	8,411,851,000		8,172,112,000		7,999,521,000
Ceded reserve at end of year	(1,339,235,000)		(1,165,022,000)		(1,017,028,000)
Gross reserves at end of year	9,751,086,000		9,337,134,000		9,016,549,000
Decrease in estimates for claims occurring in prior years, undiscounted basis	100,667,000		182,937,000		246,941,000
Decrease in estimates for claims occurring in prior years (net of additional and return premiums)	$ 103,000,000		$ 182,000,000		$ 234,000,000

[1] Claims occurring during the current year are net of loss reserve discounts of $26,078,000, $43,286,000 and $67,763,000 in 2012, 2011 and 2010, 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 $103 million, $182 million and $234 million in 2012, 2011 and 2010, 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 $100,667,000, $182,937,000 and $246,941,000 in 2012, 2011 and 2010, respectively.