Industry Segments (Financial Information of Company Operating Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Earned Premiums									$ 4,160,867		$ 3,835,582		$ 3,805,849
Investment Income									526,351		530,525		379,008
Other Revenues									468,766		357,962		246,321
Total revenues	1,370,953	1,279,731	1,270,949	1,234,351	1,231,342	1,176,112	1,163,068	1,153,547	5,155,984		4,724,069		4,431,178
Pre-Tax Income (Loss)									518,283		603,305		382,230
Net Income (Loss)	117,926	77,308	83,082	116,487	126,851	93,619	110,207	118,610	394,803		449,287		309,057
Specialty Segment [Member]
Earned Premiums									1,442,748		1,288,373		1,354,355
Investment Income									175,289		180,063		125,351
Other Revenues									2,704		3,130		3,560
Total revenues									1,620,741		1,471,566		1,483,266
Pre-Tax Income (Loss)									292,759		296,645		220,906
Net Income (Loss)									211,871		214,769		167,732
Regional Segment [Member]
Earned Premiums									1,065,975		1,066,922		1,116,871
Investment Income									75,404		82,411		57,530
Other Revenues									4,112		3,114		2,725
Total revenues									1,145,491		1,152,447		1,177,126
Pre-Tax Income (Loss)									32,382		117,353		106,078
Net Income (Loss)									30,331		86,325		80,031
Alternative Markets Segment [Member]
Earned Premiums									612,558		608,191		597,932
Investment Income									121,360		123,309		83,719
Other Revenues									86,031		79,173		87,032
Total revenues									819,949		810,673		768,683
Pre-Tax Income (Loss)									146,030		178,607		162,875
Net Income (Loss)									109,859		131,126		121,993
Reinsurance Segment [Member]
Earned Premiums									426,008		419,356		411,511
Investment Income									91,871		103,079		75,505
Other Revenues									0		0		0
Total revenues									517,879		522,435		487,016
Pre-Tax Income (Loss)									83,251		129,922		86,358
Net Income (Loss)									65,422		97,015		70,675
International Segment [Member]
Earned Premiums									613,578		452,740		325,180
Investment Income									42,882		32,794		26,767
Other Revenues									0		0		0
Total revenues									656,460		485,534		351,947
Pre-Tax Income (Loss)									40,084		21,174		22,719
Net Income (Loss)									26,055		14,838		14,676
Corporate, Other and Eliminations Segment [Member]
Earned Premiums									0	[1]	0	[1]	0	[1]
Investment Income									19,545	[1]	8,869	[1]	10,136	[1]
Other Revenues									250,438	[1]	215,964	[1]	191,412	[1]
Total revenues									269,983	[1]	224,833	[1]	201,548	[1]
Pre-Tax Income (Loss)									(201,704)	[1]	(196,977)	[1]	(178,298)	[1]
Net Income (Loss)									(130,382)	[1]	(131,660)	[1]	(121,176)	[1]
Net Investment Gains Segment [Member]
Earned Premiums									0		0		0
Investment Income									0		0		0
Other Revenues									125,481		56,581		(38,408)
Total revenues									125,481		56,581		(38,408)
Pre-Tax Income (Loss)									125,481		56,581		(38,408)
Net Income (Loss)									$ 81,647		$ 36,874		$ (24,874)

[1] Corporate, other and eliminations represent corporate revenues and expenses, net investment gains and losses and other items that are not allocated to business segments.