Industry Segments (Identifiable Assets by Segment) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Consolidated Assets	$ 20,155,896		$ 18,403,873
Specialty Segment [Member]
Consolidated Assets	6,720,056		6,157,853
Regional Segment [Member]
Consolidated Assets	2,587,603		2,488,940
Alternative Markets Segment [Member]
Consolidated Assets	4,313,404		4,044,915
Reinsurance Segment [Member]
Consolidated Assets	2,773,133		2,732,489
International Segment [Member]
Consolidated Assets	1,965,359		1,569,749
Corporate, Other and Eliminations Segment [Member]
Consolidated Assets	$ 1,796,341	[1]	$ 1,409,927	[1]

[1] Corporate, other and eliminations represent corporate revenues and expenses, net investment gains and losses and other items that are not allocated to business segments.