Industry Segments Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 1,531,558	$ 1,460,804	$ 4,496,612		$ 4,242,061
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	1,181,923	1,093,554	3,459,118		3,138,806
Insurance-Domestic Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	395,400	368,818	1,146,884		1,070,525
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	322,896	291,297	940,636		824,541
Insurance-Domestic Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	230,733	222,561	687,626	[1]	642,833	[1]
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	138,483	134,197	411,951		385,667
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	94,411	76,681	272,021		215,240
Insurance-International Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	197,578	205,529	583,041		592,721
Insurance-International Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	22,733	27,705	69,527		70,036
Insurance-International Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	24,849	18,012	69,643		52,578
Insurance-International Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	96,624	100,417	285,980	[1]	303,119	[1]
Insurance-International Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	32,177	29,076	94,278		86,318
Insurance-International Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	21,195	30,319	63,613		80,670
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	152,057	161,721	454,453		510,534
Reinsurance-Global Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	48,859	38,980	139,991		137,640
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 103,198	$ 122,741	$ 314,462		$ 372,894

[1] For the Three Months For the Nine Months Ended September 30, Ended September 30,(In thousands)2015 2014 2015 2014 Insurance-Domestic: Other liability$395,400 $368,818 $1,146,884 $1,070,525Workers’ compensation322,896 291,297 940,636 824,541Short-tail lines 230,733 222,561 687,626 642,833Commercial automobile138,483 134,197 411,951 385,667Professional liability94,411 76,681 272,021 215,240Total1,181,923 1,093,554 3,459,118 3,138,806 Insurance-International: Other liability22,733 27,705 69,527 70,036Workers’ compensation24,849 18,012 69,643 52,578Short-tail lines96,624 100,417 285,980 303,119Commercial automobile32,177 29,076 94,278 86,318Professional liability21,195 30,319 63,613 80,670Total197,578 205,529 583,041 592,721 Reinsurance-Global: Casualty103,198 122,741 314,462 372,894Property48,859 38,980 139,991 137,640Total152,057 161,721 454,453 510,534 Total$1,531,558 $1,460,804 $4,496,612 $4,242,061