Segment Information (Details) - Net premiums earned by line of business (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
P&C Operations:
P&C Operations	$ 1,400,972		$ 1,306,125		$ 2,758,870		$ 2,577,821
Life Operations:
Life Operations	85,623		92,214		164,143		181,901
Total	1,486,595		1,398,339		2,923,013		2,759,722
Life Operations Segment [Member]
Life Operations:
Life Operations	85,623		92,214		164,143		181,901
Total	85,623		92,214		164,143		181,901
Total P And C Segment [Member]
Life Operations:
Total	1,400,972		1,306,125		2,758,870		2,577,821
Insurance Segment [Member]
P&C Operations:
P&C Operations	959,294		907,443		1,893,350		1,783,363
Life Operations:
Total	959,294		907,443		1,893,350		1,783,363
Reinsurance Segment [Member]
P&C Operations:
P&C Operations	441,678		398,682		865,520		794,458
Life Operations:
Total	441,678		398,682		865,520		794,458
Casualty Professional Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	384,179		374,569		762,905		743,929
Casualty Professional Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	330,876		322,654		654,696		638,297
Casualty Professional Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	53,303		51,915		108,209		105,632
Casualty Other Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	244,404		241,166		485,000		448,913
Casualty Other Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	167,562		172,246		334,920		328,950
Casualty Other Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	76,842		68,920		150,080		119,963
Property Catastrophe [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	113,221		94,363		216,146		188,927
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	113,221		94,363		216,146		188,927
Other Property [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	269,918		252,343		539,081		495,507
Other Property [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	124,893		117,088		258,312		224,009
Other Property [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	145,025		135,255		280,769		271,498
Marine Energy Aviation And Satellite [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	179,608		158,794		335,452		322,757
Marine Energy Aviation And Satellite [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	143,981		128,358		264,081		253,893
Marine Energy Aviation And Satellite [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	35,627		30,436		71,371		68,864
Other Speciality Lines [Member] | Total P And C Segment [Member]
P&C Operations:
P&C Operations	190,169	[1]	165,027	[1]	374,203	[1]	333,693	[1]
Other Speciality Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	190,169	[1]	165,027	[1]	374,203	[1]	333,693	[1]
Other Speciality 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	19,473	[2]	19,863	[2]	46,083	[2]	44,095	[2]
Other Lines [Member] | Insurance Segment [Member]
P&C Operations:
P&C Operations	1,813	[2]	2,070	[2]	7,138	[2]	4,521	[2]
Other Lines [Member] | Reinsurance Segment [Member]
P&C Operations:
P&C Operations	17,660	[2]	17,793	[2]	38,945	[2]	39,574	[2]
Other Life [Member]
Life Operations:
Life Operations	54,380		58,246		101,443		115,010
Other Life [Member] | Life Operations Segment [Member]
Life Operations:
Life Operations	54,380		58,246		101,443		115,010
Annuity [Member]
Life Operations:
Life Operations	31,243		33,968		62,700		66,891
Annuity [Member] | Life Operations Segment [Member]
Life Operations:
Life Operations	$ 31,243		$ 33,968		$ 62,700		$ 66,891

[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.