Reserves For Losses And Loss Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves	$ 7,999,521,000		$ 8,147,782,000		$ 8,122,586,000
Net provision for losses and loss expenses:
Claim occurring during the current year	2,791,860,000	[1]	2,509,933,000	[1]	2,518,849,000	[1]
Decrease in estimates for claims occurring in prior years	(181,282,000)	[2],[3]	(253,248,000)	[2],[3]	(234,008,000)	[2],[3]
Loss reserve discount accretion	47,787,000		53,182,000		51,866,000
Total net incurred	2,658,365,000		2,309,867,000		2,336,707,000
Net payments for claims:
Current year	765,440,000		641,570,000		582,605,000
Prior year	1,721,558,000		1,811,507,000		1,751,026,000
Total net paid	2,486,998,000		2,453,077,000		2,333,631,000
Foreign currency translation	1,224,000		(5,051,000)		22,120,000
Net reserves	8,172,112,000		7,999,521,000		8,147,782,000
Ceded reserve at end of year	(1,165,022,000)		(1,017,028,000)		(923,889,000)
Gross reserves at end of year	9,337,134,000		9,016,549,000		9,071,671,000
Workers' compensation discount, amount	43,286,000		67,763,000		80,455,000
Decrease in estimates for claims occurring in prior years, undiscounted basis	182,937,000		246,941,000		232,040,000
Decrease in estimates for claims occurring in prior years (net of additional and return premiums)	$ 182,000,000		$ 234,000,000		$ 190,000,000

[1] Claims occurring during the current year are net of loss reserve discounts of $43,286,000, $67,763,000 and $80,455,000 in 2011, 2010 and 2009, 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 $182,937,000, $246,941,000 and $232,040,000 in 2011, 2010 and 2009, respectively.

[3] 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 $182 million, $234 million and $190 million in 2011, 2010 and 2009, respectively.