Industry Segments (Identifiable Assets by Segment) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Consolidated Assets	$ 19,133,632		$ 18,403,873
Specialty Segment [Member]
Consolidated Assets	6,223,195		6,157,853
Regional Segment [Member]
Consolidated Assets	2,520,652		2,488,940
Alternative Markets Segment [Member]
Consolidated Assets	4,166,892		4,044,915
Reinsurance Segment [Member]
Consolidated Assets	2,761,580		2,732,489
International Segment [Member]
Consolidated Assets	1,700,414		1,569,749
Corporate, Other and Eliminations Segment [Member]
Consolidated Assets	$ 1,760,899	[1]	$ 1,409,927	[1]

[1] Corporate and eliminations represent corporate revenues and expenses that are not allocated to business segments.