Investments (Schedule Of Trading Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	$ 7,125,860		$ 6,254,686
Fixed Maturities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Amortized Cost	6,934,821		6,207,991
U.S. Government And Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,849,492		1,278,265
Trading securities, Amortized Cost	1,831,521		1,263,948
Non-U.S. Government And Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	309,471		256,756
Trading securities, Amortized Cost	297,424		251,784
States, Municipalities And Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	41,542		133,902
Trading securities, Amortized Cost	40,327		128,633
Corporate Debt, Financial Institutions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,031,764		1,161,904
Trading securities, Amortized Cost	997,781		1,174,308
Corporate Debt, Industrials [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,145,621		987,006
Trading securities, Amortized Cost	1,128,154		974,731
Corporate Debt, Utilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	59,802		105,564
Trading securities, Amortized Cost	57,622		103,262
Residential Mortgage-Backed, Non-Agency Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	337,662		302,827
Trading securities, Amortized Cost	287,354		314,077
Residential Mortgage-Backed, Agency Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,606,201		1,183,893
Trading securities, Amortized Cost	1,568,320		1,156,913
Commercial Mortgage-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	308,094		331,371
Trading securities, Amortized Cost	294,901		326,697
Asset-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	436,211		513,198
Trading securities, Amortized Cost	431,417		513,638
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	490,418		367,483
Trading securities, Amortized Cost	436,983		356,370
Other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	564,702	[1]	540,409	[1]
Trading securities, Amortized Cost	520,233	[1]	529,851	[1]
Equity securities and other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,055,120		907,892
Trading securities, Amortized Cost	$ 957,216		$ 886,221

[1] Within the Company's financial statements and footnotes "other invested assets" include the Company's investments in both hedge funds and private equity funds.