Derivative Instruments (Derivative Fair Values on the Condensed Consolidated Balance Sheets) (Details) (Derivatives Not Designated as Hedging Instruments [Member], USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Investment Portfolio [Member]
Asset Derivative Notional Amount	$ 1,919,511
Asset Derivative Fair Value	5,670
Liability Derivative Notional Amount	176,173
Liability Derivative Fair Value	236
Investment Portfolio [Member] | Foreign Exchange Contracts [Member]
Asset Derivative Notional Amount	122,311	[1]
Asset Derivative Fair Value	1,487	[1]
Liability Derivative Notional Amount	33,873	[1]
Liability Derivative Fair Value	204	[1]
Investment Portfolio [Member] | Interest Rate Futures [Member]
Asset Derivative Notional Amount	1,797,200	[1]
Asset Derivative Fair Value	4,183	[1]
Liability Derivative Notional Amount	142,300	[1]
Liability Derivative Fair Value	32	[1]
Operating Activities [Member]
Asset Derivative Notional Amount	2,045,039		26,758
Asset Derivative Fair Value	7,048		858
Liability Derivative Notional Amount	211,003		51,308
Liability Derivative Fair Value	654		1,629
Operating Activities [Member] | Foreign Exchange Contracts [Member]
Asset Derivative Notional Amount	125,528	[2]	26,758	[2]
Asset Derivative Fair Value	1,378	[2]	858	[2]
Liability Derivative Notional Amount	34,830	[2]	51,308	[2]
Liability Derivative Fair Value	$ 418	[2]	$ 1,629	[2]

[1] Asset and liability derivatives relating to the investment portfolio are classified within "balances receivable on sale of investments" or "balances due on the purchase of investments" on the consolidated balance sheets.

[2] Asset and liability derivatives relating to operating activities are classified within "other assets" or "other liabilities" on the consolidated balance sheets.