Derivative Instruments (Schedule Of Derivative Fair Values On Condensed Consolidated Balance Sheets) (Details) (Derivatives Not Designated As Hedging Instruments [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	$ 776,273		$ 26,758
Asset Derivative Fair Value	3,343		858
Liability Derivative Notional Amount	631,533		51,308
Liability Derivative Fair Value	12,401		1,629
Investment Portfolio [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	749,691
Asset Derivative Fair Value	3,067
Liability Derivative Notional Amount	525,801
Liability Derivative Fair Value	8,655
Investment Portfolio [Member] | Put Options [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	4,461	[1]
Asset Derivative Fair Value	336	[1]
Investment Portfolio [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	64,580	[2]
Asset Derivative Fair Value	1,754	[2]
Liability Derivative Notional Amount	233,801	[2]
Liability Derivative Fair Value	5,188	[2]
Investment Portfolio [Member] | Interest Rate Futures [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	680,650	[2]
Asset Derivative Fair Value	977	[2]
Liability Derivative Notional Amount	292,000	[2]
Liability Derivative Fair Value	3,467	[2]
Operating Activities [Member] | Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	26,582	[2]	26,758	[2]
Asset Derivative Fair Value	276	[2]	858	[2]
Liability Derivative Notional Amount	105,732	[2]	51,308	[2]
Liability Derivative Fair Value	$ 3,746	[2]	$ 1,629	[2]

[1] Asset and liability derivatives relating to the put options are classified within "equity securities trading, at fair value" on the consolidated balance sheets.

[2] All other asset and liability derivatives are classified within "other assets" or "accounts payable and accrued liabilities" on the consolidated balance sheets.