Industry Segments Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 1,417,645	$ 1,282,515	$ 2,781,257		$ 2,514,634
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	1,041,745	925,997	2,045,252		1,810,375
Insurance-Domestic Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	362,074	307,444	701,707		603,032
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	270,212	241,191	533,244		470,202
Insurance-Domestic Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	215,506	192,938	420,272	[1]	371,687	[1]
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	127,102	119,900	251,470		236,022
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	66,851	64,524	138,559		129,432
Insurance-International Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	201,868	180,192	387,192		351,311
Insurance-International Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	22,956	15,070	42,331		30,288
Insurance-International Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	17,241	21,746	34,566		44,635
Insurance-International Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	106,428	84,301	202,702	[1]	156,424	[1]
Insurance-International Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	29,649	32,392	57,242		67,031
Insurance-International Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	25,594	26,683	50,351		52,933
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	174,032	176,326	348,813		352,948
Reinsurance-Global Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	49,482	54,209	98,660		109,504
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 124,550	$ 122,117	$ 250,153		$ 243,444

[1] For the Three MonthsFor the Six Months Ended June 30,Ended June 30,(In thousands)2014 20132014 2013 Insurance-Domestic: Other liability$362,074$307,444$701,707 $603,032Workers’ compensation270,212241,191533,244 470,202Short-tail lines 215,506192,938420,272 371,687Commercial automobile127,102119,900251,470 236,022Professional liability66,85164,524138,559 129,432Total1,041,745925,9972,045,252 1,810,375 Insurance-International: Other liability22,95615,07042,331 30,288Workers’ compensation17,24121,74634,566 44,635Short-tail lines106,42884,301202,702 156,424Commercial automobile29,64932,39257,242 67,031Professional liability25,59426,68350,351 52,933Total201,868180,192387,192 351,311 Reinsurance-Global: Casualty124,550122,117250,153 243,444Property49,48254,20998,660 109,504Total174,032176,326348,813 352,948 Total$1,417,645 $1,282,515$2,781,257 $2,514,634