Industry Segments (Financial Information of Company Operating Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Earned Premiums									$ 4,673,516		$ 4,160,867		$ 3,835,582
Investment Income									586,763		526,351		530,525
Other Revenues									563,275		468,766		357,962
Total revenues	1,608,601	1,420,501	1,415,747	1,378,705	1,370,953	1,279,731	1,270,949	1,234,351	5,823,554		5,155,984		4,724,069
Pre-Tax Income (Loss)									701,928		513,086		598,910
Net Income (Loss)	165,489	100,947	108,838	135,318	117,027	76,410	82,184	115,590	510,592		391,211		446,405
Specialty Segment [Member]
Earned Premiums									1,646,471		1,442,748		1,288,373
Investment Income									186,998		175,289		180,063
Other Revenues									2,348		2,704		3,130
Total revenues									1,835,817		1,620,741		1,471,566
Pre-Tax Income (Loss)									261,856		290,937		294,825
Net Income (Loss)									186,270		210,687		213,586
Regional Segment [Member]
Earned Premiums									1,090,217		1,065,975		1,066,922
Investment Income									81,635		75,404		82,411
Other Revenues									6,406		4,112		3,114
Total revenues									1,178,258		1,145,491		1,152,447
Pre-Tax Income (Loss)									122,211		30,529		115,828
Net Income (Loss)									86,450		29,127		85,334
Alternative Markets Segment [Member]
Earned Premiums									680,334		612,558		608,191
Investment Income									156,154		121,360		123,309
Other Revenues									94,379		86,031		79,173
Total revenues									930,867		819,949		810,673
Pre-Tax Income (Loss)									194,433		145,660		178,485
Net Income (Loss)									139,795		109,618		131,047
Reinsurance Segment [Member]
Earned Premiums									446,939		426,008		419,356
Investment Income									96,123		91,871		103,079
Other Revenues									0		0		0
Total revenues									543,062		517,879		522,435
Pre-Tax Income (Loss)									93,268		83,150		129,449
Net Income (Loss)									68,881		65,356		96,708
International Segment [Member]
Earned Premiums									809,555		613,578		452,740
Investment Income									56,605		42,882		32,794
Other Revenues									0		0		0
Total revenues									866,160		656,460		485,534
Pre-Tax Income (Loss)									62,061		39,033		20,719
Net Income (Loss)									25,390		25,158		14,516
Corporate, Other and Eliminations Segment [Member]
Earned Premiums									0	[1]	0	[1]	0	[1]
Investment Income									9,248	[1]	19,545	[1]	8,869	[1]
Other Revenues									249,677	[1]	250,438	[1]	215,964	[1]
Total revenues									258,925	[1]	269,983	[1]	224,833	[1]
Pre-Tax Income (Loss)									(242,366)	[1]	(201,704)	[1]	(196,977)	[1]
Net Income (Loss)									(132,996)	[1]	(130,382)	[1]	(131,660)	[1]
Net Investment Gains Segment [Member]
Earned Premiums									0		0		0
Investment Income									0		0		0
Other Revenues									210,465		125,481		56,581
Total revenues									210,465		125,481		56,581
Pre-Tax Income (Loss)									210,465		125,481		56,581
Net Income (Loss)									$ 136,802		$ 81,647		$ 36,874

[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.