Investments In Securities And Interest Rate Swap And Cap Contracts (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Trading Securities and Other Trading Assets [Line Items]
Weighted average expected life of agency securities (in years)	5
Investments in securities, pledged assets	$ 8,412,295,000	$ 3,671,582,000
Agency RMBS [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unamortized premium on marketable securities	223,500,000	152,700,000
Actual maturities of Agency RMBS, range, (in years)	30 years
Interest income on Agency RMBS	228,800,000	72,700,000	44,000,000
Securities maturity date	2031
Investments in securities, pledged assets		10,100,000
Agency RMBS And U.S Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments in securities, pledged assets	$ 127,900,000	$ 4,300,000