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, 2013		Jun. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,328,862		$ 1,186,533		$ 3,843,496		$ 3,433,541
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned	958,994		873,835		2,769,369		2,530,638
Insurance-Domestic Segment [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned	314,878		280,808		917,910		812,153
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned	252,220		223,477		722,422		637,583
Insurance-Domestic Segment [Member] | Short-tail lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned	199,181	[1]	189,613	[1]	570,868	[1]	552,905	[1]
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned	125,942		116,418		361,964		339,317
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned	66,773		63,519		196,205		188,680
Insurance-International Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned	189,054		157,773		540,365		456,159
Insurance-International Segment [Member] | Other Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned	18,457		29,719		48,745		47,559
Insurance-International Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned	20,847		21,102		65,482		59,618
Insurance-International Segment [Member] | Short-tail lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned	92,213	[1]	52,043	[1]	248,637	[1]	183,791	[1]
Insurance-International Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned	30,898		30,557		97,929		90,831
Insurance-International Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned	26,639		24,352		79,572		74,360
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned	180,814		154,925		533,762		446,744
Reinsurance-Global Segment [Member] | Short-tail lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned	52,687		42,011		162,191		125,467
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Net premiums earned	$ 128,127		$ 112,914		$ 371,571		$ 321,277

[1] Short-tail lines includes commercial multi-peril (non-liability), inland and ocean marine, accident and health, fidelity and surety, boiler and machinery and other lines.