Industry Segments Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 1,493,041	$ 1,417,645	$ 2,965,054		$ 2,781,257
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	1,159,653	1,041,745	2,277,195		2,045,252
Insurance-Domestic Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	379,445	362,074	751,484		701,707
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	325,123	270,212	617,740		533,244
Insurance-Domestic Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	229,167	215,506	456,893	[1]	420,272	[1]
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	138,511	127,102	273,468		251,470
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	87,407	66,851	177,610		138,559
Insurance-International Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	191,729	201,868	385,463		387,192
Insurance-International Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	21,929	22,956	46,794		42,331
Insurance-International Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	22,673	17,241	44,794		34,566
Insurance-International Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	93,992	106,428	189,356	[1]	202,702	[1]
Insurance-International Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	31,498	29,649	62,101		57,242
Insurance-International Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	21,637	25,594	42,418		50,351
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	141,659	174,032	302,396		348,813
Reinsurance-Global Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	43,211	49,482	91,132		98,660
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 98,448	$ 124,550	$ 211,264		$ 250,153

[1] For the Three Months For the Six Months Ended June 30, Ended June 30,(In thousands)2015 2014 2015 2014 Insurance-Domestic: Other liability$379,445 $362,074 $751,484 $701,707Workers’ compensation325,123 270,212 617,740 533,244Short-tail lines 229,167 215,506 456,893 420,272Commercial automobile138,511 127,102 273,468 251,470Professional liability87,407 66,851 177,610 138,559Total1,159,653 1,041,745 2,277,195 2,045,252 Insurance-International: Other liability21,929 22,956 46,794 42,331Workers’ compensation22,673 17,241 44,794 34,566Short-tail lines93,992 106,428 189,356 202,702Commercial automobile31,498 29,649 62,101 57,242Professional liability21,637 25,594 42,418 50,351Total191,729 201,868 385,463 387,192 Reinsurance-Global: Casualty98,448 124,550 211,264 250,153Property43,211 49,482 91,132 98,660Total141,659 174,032 302,396 348,813 Total$1,493,041 $1,417,645 $2,965,054 $2,781,257