Investments In Securities And Interest Rate Swap And Cap Contracts (Summary Of Interest Rate Swap And Cap Contracts) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	Net Increase	Net Increase
Notional Amount of Interest Rate Derivatives	$ 1,050,000	$ 3,650,000
February 2011 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	February 2011
Notional Amount of Interest Rate Derivatives	500,000
March 2011 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	March 2011
Notional Amount of Interest Rate Derivatives	250,000
May 2011 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	May 2011
Notional Amount of Interest Rate Derivatives	(300,000)
May 2011 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	May 2011
Notional Amount of Interest Rate Derivatives	600,000
October 2011 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	October 2011
Notional Amount of Interest Rate Derivatives	(250,000)
December 2011 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date	December 2011
Notional Amount of Interest Rate Derivatives	250,000
April 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		April 2010
Notional Amount of Interest Rate Derivatives		(640,000)
April 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		April 2010
Notional Amount of Interest Rate Derivatives		400,000
May 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		May 2010
Notional Amount of Interest Rate Derivatives		740,000
May 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		May 2010
Notional Amount of Interest Rate Derivatives		(400,000)
June 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		June 2010
Notional Amount of Interest Rate Derivatives		400,000
June 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		June 2010
Notional Amount of Interest Rate Derivatives		(100,000)
July 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		July 2010
Notional Amount of Interest Rate Derivatives		750,000
August 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		August 2010
Notional Amount of Interest Rate Derivatives		200,000
August 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		August 2010
Notional Amount of Interest Rate Derivatives		(200,000)
September 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		September 2010
Notional Amount of Interest Rate Derivatives		500,000
October 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		October 2010
Notional Amount of Interest Rate Derivatives		(240,000)
October 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		October 2010
Notional Amount of Interest Rate Derivatives		540,000
November 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		November 2010
Notional Amount of Interest Rate Derivatives		200,000
November 2010 Terminated Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		November 2010
Notional Amount of Interest Rate Derivatives		(200,000)
December 2010 Opened Type [Member]
Summary Of Interest Rate Swap And Cap Contracts [Line Items]
Trade Date		December 2010
Notional Amount of Interest Rate Derivatives		$ 1,700,000