Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net premiums earned	$ 1,232,119	$ 1,099,651	$ 4,673,516	$ 4,160,867	$ 3,835,582
Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	884,378	810,069	3,417,022	3,121,281	2,963,486
Insurance-International Segment [Member] [Domain]
Net premiums earned	171,119	143,885	631,841	508,509	378,693
Reinsurance-Global Segment [Member] [Domain]
Net premiums earned	176,622	145,697	624,653	531,077	493,403
Property [Member] | Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	178,749	173,902
Property [Member] | Insurance-International Segment [Member] [Domain]
Net premiums earned	72,123	52,478
Property [Member] | Reinsurance-Global Segment [Member] [Domain]
Net premiums earned	55,295	40,608
Commercial Automobile [Member] | Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	116,122	110,367
Commercial Automobile [Member] | Insurance-International Segment [Member] [Domain]
Net premiums earned	34,639	30,668
Products Liability [Member] | Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	64,908	62,692
Products Liability [Member] | Insurance-International Segment [Member] [Domain]
Net premiums earned	26,250	26,587
Workers' Compensation [Member] | Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	229,011	189,544
Workers' Compensation [Member] | Insurance-International Segment [Member] [Domain]
Net premiums earned	22,889	18,618
Casualty [Member] | Reinsurance-Global Segment [Member] [Domain]
Net premiums earned	121,327	105,089
Other Liability [Member] | Insurance-Domestic Segment [Member] [Domain]
Net premiums earned	295,588	273,564
Other Liability [Member] | Insurance-International Segment [Member] [Domain]
Net premiums earned	$ 15,218	$ 15,534