Segment Information (Details) - Net premiums earned by line of business (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
P&C Operations:
P&C Operations	$ 1,468,553		$ 1,362,532		$ 4,227,423		$ 3,940,353
Life Operations:
Life Operations	79,487		90,794		243,630		272,695
Total	1,548,040		1,453,326		4,471,053		4,213,048
Life Operations Segment [Member]
Life Operations:
Life Operations	79,487		90,794		243,630		272,695
Total	79,487		90,794		243,630		272,695
Total P And C Segment [Member]
Life Operations:
Total	1,468,553		1,362,532		4,227,423		3,940,353
Insurance Segment [Member]
P&C Operations:
P&C Operations	985,623		930,067		2,878,973		2,713,430
Life Operations:
Total	985,623		930,067		2,878,973		2,713,430
Reinsurance Segment [Member]
P&C Operations:
P&C Operations	482,930		432,465		1,348,450		1,226,923
Life Operations:
Total	482,930		432,465		1,348,450		1,226,923
Casualty Professional Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	393,596		371,046		1,156,499		1,114,975
Casualty Professional Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	345,722		319,101		1,000,417		957,399
Casualty Professional Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	47,874		51,945		156,082		157,576
Casualty Other Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	263,063		236,958		748,064		685,872
Casualty Other Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	181,265		172,684		516,185		501,634
Casualty Other Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	81,798		64,274		231,879		184,238
Property Catastrophe [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	113,167		97,158		329,313		286,085
Property Catastrophe [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	0		0		0		0
Property Catastrophe [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	113,167		97,158		329,313		286,085
Other Property [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	311,714		281,979		850,795		777,486
Other Property [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	140,858		120,394		399,170		344,403
Other Property [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	170,856		161,585		451,625		433,083
Marine Energy Aviation And Satellite [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	163,588		170,452		499,040		493,209
Marine Energy Aviation And Satellite [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	130,497		138,290		394,578		392,183
Marine Energy Aviation And Satellite [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	33,091		32,162		104,462		101,026
Other Specialty Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	185,622	[1]	178,706	[1]	559,826	[1]	512,399	[1]
Other Specialty Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	185,622	[1]	178,706	[1]	559,826	[1]	512,399	[1]
Other Specialty Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	0	[1]	0	[1]	0	[1]	0	[1]
Other Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	37,803	[2]	26,233	[2]	83,886	[2]	70,327	[2]
Other Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	1,659	[2]	892	[2]	8,797	[2]	5,412	[2]
Other Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	36,144	[2]	25,341	[2]	75,089	[2]	64,915	[2]
Annuity [Member]
Life Operations:
Life Operations	31,960		33,342		94,660		100,232
Annuity [Member] | Life Operations Segment [Member]
Life Operations:
Life Operations	31,960		33,342		94,660		100,232
Other Life [Member]
Life Operations:
Life Operations	47,527		57,452		148,970		172,463
Other Life [Member] | Life Operations Segment [Member]
Life Operations:
Life Operations	$ 47,527		$ 57,452		$ 148,970		$ 172,463

[1] Other specialty lines within the Insurance segment includes: environmental, programs, equine, warranty, specie, middle markets and excess and surplus lines.

[2] Other includes credit and surety, whole account contracts, structured indemnity and other lines.