Industry Segments (Identifiable Assets by Segment) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Consolidated Assets	$ 20,120,634		$ 20,155,896		$ 18,403,873
Insurance-Domestic Segment [Member] [Domain]
Consolidated Assets	14,023,370		14,661,476		13,126,472
Insurance-International Segment [Member] [Domain]
Consolidated Assets	1,690,937		1,605,448		1,341,907
Reinsurance-Global Segment [Member] [Domain]
Consolidated Assets	3,420,508		3,337,937		3,168,528
Corporate, Other and Eliminations Segment [Member]
Consolidated Assets	$ 985,819	[1]	$ 615,118	[1]	$ 766,966	[1]

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