Industry Segments (Financial Information of Company Operating Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Earned Premiums	$ 1,232,119		$ 1,099,651		$ 4,673,516		$ 4,160,867		$ 3,835,582
Investment Income and Funds	135,929		157,619		586,763		526,351		530,525
Other Revenues	138,721		121,435		563,275		468,766		357,962
Total revenues	1,506,769		1,378,705		5,823,554		5,155,984		4,724,069
Pre-Tax Income (Loss)	160,223		187,303		701,928		513,086		598,910
Net Income (Loss)	116,615		135,318		510,592		391,211		446,405
Insurance-Domestic Segment [Member] [Domain]
Earned Premiums	884,378		810,069		3,417,022		3,121,281		2,963,486
Investment Income and Funds	97,797		112,086		424,787		372,053		385,783
Other Revenues	26,736		23,877		103,133		92,847		85,417
Total revenues	1,008,911		946,032		3,944,942		3,586,181		3,434,686
Pre-Tax Income (Loss)	141,350		147,735		578,500		467,126		589,138
Net Income (Loss)	99,408		105,880		412,514		344,521		425,337
Insurance-International Segment [Member] [Domain]
Earned Premiums	171,119		143,885		631,841		508,509		378,693
Investment Income and Funds	12,061		10,331		45,796		36,958		29,011
Other Revenues	0		0		0		0		0
Total revenues	183,180		154,216		677,637		545,467		407,704
Pre-Tax Income (Loss)	22,382		15,699		51,639		36,912		15,172
Net Income (Loss)	15,477		11,112		37,499		28,054		13,050
Reinsurance-Global Segment [Member] [Domain]
Earned Premiums	176,622		145,697		624,653		531,077		493,403
Investment Income and Funds	22,499		28,687		106,932		97,795		106,862
Other Revenues	0		0		0		0		0
Total revenues	199,121		174,384		731,585		628,872		600,265
Pre-Tax Income (Loss)	37,941		31,638		103,690		85,271		134,996
Net Income (Loss)	26,394		23,086		76,584		66,174		99,492
Corporate, Other and Eliminations Segment [Member]
Earned Premiums	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Investment Income and Funds	3,572	[1]	6,515	[1]	9,248	[1]	19,545	[1]	8,869	[1]
Other Revenues	92,016	[1]	50,067	[1]	249,677	[1]	250,438	[1]	215,964	[1]
Total revenues	95,588	[1]	56,582	[1]	258,925	[1]	269,983	[1]	224,833	[1]
Pre-Tax Income (Loss)	(61,419)	[1]	(55,260)	[1]	(242,366)	[1]	(201,704)	[1]	(196,977)	[1]
Net Income (Loss)	(37,644)	[1]	(35,842)	[1]	(152,807)	[1]	(129,185)	[1]	(128,348)	[1]
Net Investment Gains Segment [Member]
Earned Premiums	0		0		0		0		0
Investment Income and Funds	0		0		0		0		0
Other Revenues	19,969		47,491		210,465		125,481		56,581
Total revenues	19,969		47,491		210,465		125,481		56,581
Pre-Tax Income (Loss)	19,969		47,491		210,465		125,481		56,581
Net Income (Loss)	$ 12,980		$ 31,082		$ 136,802		$ 81,647		$ 36,874

[1] Corporate and eliminations represent corporate revenues and expenses that are not allocated to business segments.