Industry Segments Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 1,460,804	$ 1,328,862	$ 4,242,061		$ 3,843,496
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	1,093,554	958,994	3,138,806		2,769,369
Insurance-Domestic Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	368,818	314,878	1,070,525		917,910
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	291,297	252,220	824,541		722,422
Insurance-Domestic Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	222,561	199,181	642,833	[1]	570,868	[1]
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	134,197	125,942	385,667		361,964
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	76,681	66,773	215,240		196,205
Insurance-International Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	205,529	189,054	592,721		540,365
Insurance-International Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	27,705	18,457	70,036		48,745
Insurance-International Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	18,012	20,847	52,578		65,482
Insurance-International Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	100,417	92,213	303,119	[1]	248,637	[1]
Insurance-International Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	29,076	30,898	86,318		97,929
Insurance-International Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	30,319	26,639	80,670		79,572
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	161,721	180,814	510,534		533,762
Reinsurance-Global Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	38,980	52,687	137,640		162,191
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 122,741	$ 128,127	$ 372,894		$ 371,571

[1] For the Three Months For the Nine Months Ended September 30, Ended September 30,(In thousands)2014 2013 2014 2013 Insurance-Domestic: Other liability$368,818 $314,878 $1,070,525 $917,910Workers’ compensation291,297 252,220 824,541 722,422Short-tail lines 222,561 199,181 642,833 570,868Commercial automobile134,197 125,942 385,667 361,964Professional liability76,681 66,773 215,240 196,205Total1,093,554 958,994 3,138,806 2,769,369 Insurance-International: Other liability27,705 18,457 70,036 48,745Workers’ compensation18,012 20,847 52,578 65,482Short-tail lines100,417 92,213 303,119 248,637Commercial automobile29,076 30,898 86,318 97,929Professional liability30,319 26,639 80,670 79,572Total205,529 189,054 592,721 540,365 Reinsurance-Global: Casualty122,741 128,127 372,894 371,571Property38,980 52,687 137,640 162,191Total161,721 180,814 510,534 533,762 Total$1,460,804 $1,328,862 $4,242,061 $3,843,496