Mortgage Securities And Similar Investments (Schedule Of Mortgage Securities And Similar Investments) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010
Principal Balance	$ 10,928,689		$ 8,168,926
Investment Premiums	284,651		166,391
Basis	11,213,340		8,335,317
Carrying Amount	11,424,161	[1]	8,515,691	[1]
Net WAC	3.05%	[2]	3.10%	[2]
Average Yield	2.38%	[2]	2.51%	[2]
Fixed-Rate [Member] | Agency Securities [Member]
Principal Balance	4,386		4,927
Investment Premiums	13		14
Basis	4,399		4,941
Carrying Amount	4,410	[1]	4,955	[1]
Net WAC	6.77%	[2]	6.78%	[2]
Average Yield	6.49%	[2]	6.60%	[2]
Fixed-Rate [Member] | Residential Mortgage Loans [Member]
Principal Balance	3,351		3,455
Investment Premiums	6		6
Basis	3,357		3,461
Carrying Amount	3,357	[1]	3,461	[1]
Net WAC	6.97%	[2]	6.98%	[2]
Average Yield	6.50%	[2]	7.03%	[2]
ARMs [Member] | Agency Securities [Member]
Principal Balance	9,705,239		7,660,269
Investment Premiums	251,605		158,260
Basis	9,956,844		7,818,529
Carrying Amount	10,154,164	[1]	7,991,122	[1]
Net WAC	3.03%	[2]	3.09%	[2]
Average Yield	2.36%	[2]	2.49%	[2]
ARMs [Member] | Residential Mortgage Loans [Member]
Principal Balance	6,118		7,235
Investment Premiums	23		24
Basis	6,141		7,259
Carrying Amount	6,141	[1]	7,259	[1]
Net WAC	3.49%	[2]	3.45%	[2]
Average Yield	3.98%	[2]	3.79%	[2]
Ginnie Mae ARMs [Member] | Agency Securities [Member]
Principal Balance	1,206,258		489,579
Investment Premiums	32,949		8,030
Basis	1,239,207		497,609
Carrying Amount	1,252,697	[1]	505,376	[1]
Net WAC	3.20%	[2]	3.12%	[2]
Average Yield	2.54%	[2]	2.57%	[2]
Agency Securities [Member]
Principal Balance	10,915,883		8,154,775
Investment Premiums	284,567		166,304
Basis	11,200,450		8,321,079
Carrying Amount	11,411,271	[1]	8,501,453	[1]
Net WAC	3.05%	[2]	3.09%	[2]
Average Yield	2.38%	[2]	2.50%	[2]
Residential Mortgage Loans [Member]
Principal Balance	9,469		10,690
Investment Premiums	29		30
Basis	9,498		10,720
Carrying Amount	9,498	[1]	10,720	[1]
Net WAC	4.72%	[2]	4.59%	[2]
Average Yield	4.84%	[2]	4.83%	[2]
Senior Notes [Member]
Principal Balance
Investment Premiums
Basis
Carrying Amount				[1]
Net WAC				[2]
Average Yield			8.75%	[2]
Structured Financings Secured By Private Residential Mortgage Securities [Member]
Principal Balance	3,337		3,461
Investment Premiums	55		57
Basis	3,392		3,518
Carrying Amount	$ 3,392	[1]	$ 3,518	[1]
Net WAC	8.04%	[2]	7.97%	[2]
Average Yield	7.88%	[2]	8.07%	[2]

[1] Includes unrealized gains and losses for securities classified as available-for-sale, if applicable, (see NOTE 9).

[2] Net WAC is presented net of servicing and other fees and represents the cash yield inherent in the portfolio as of the indicated balance sheet date, before amortization of investment premiums. Average yield is presented for the quarter then ended, calculated including the amortization of investment premiums and excluding unrealized gains and losses.