Accumulated Other Comprehensive Income (Components Of Accumulated Other Comprehensive Income) (Details) (USD $) In Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Accumulated Other Comprehensive Income [Abstract]
Unrealized gain on agency securities, net	$ 345,153		$ 162,312
Unrealized gain (loss) on unsettled securities	17		(64)
Unrealized loss on short term investment	(154)	[1]	(136)	[1]
Unrealized loss on derivative instruments	(227,821)		(56,221)
Accumulated other comprehensive income	117,195		105,891
Unrealized gains and losses on available-for-sale securities	$ 17,735		$ 13,551

[1] This line item is included in Other Assets on the Balance Sheet.