Industry Segments (Identifiable Assets by Segment) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Consolidated Assets	$ 18,487,731		$ 17,528,547
Specialty Segment [Member]
Consolidated Assets	6,180,996		5,854,256
Regional Segment [Member]
Consolidated Assets	2,534,875		2,616,238
Alternative Markets Segment [Member]
Consolidated Assets	4,050,949		3,801,597
Reinsurance Segment [Member]
Consolidated Assets	2,734,277		2,972,988
International Segment [Member]
Consolidated Assets	1,576,707		1,391,604
Corporate, Other and Eliminations Segment [Member]
Consolidated Assets	$ 1,409,927	[1]	$ 891,864	[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.