Mortgage Securities And Similar Investments (Schedule Of Mortgage Securities And Similar Investments) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	$ 11,682,982		$ 8,168,926
Investment Premiums	320,179		166,391
Basis	12,003,161		8,335,317
Carrying Amount	12,240,100	[1]	8,515,691	[1]
Net WAC	2.93%	[2]	3.10%	[2]
Average Yield	2.17%	[2]	2.51%	[2]
Fixed-Rate [Member] | Agency Securities [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	4,260		4,927
Investment Premiums	12		14
Basis	4,272		4,941
Carrying Amount	4,280	[1]	4,955	[1]
Net WAC	6.74%	[2]	6.78%	[2]
Average Yield	6.66%	[2]	6.60%	[2]
Fixed-Rate [Member] | Residential Mortgage Loans [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	3,291		3,455
Investment Premiums	5		6
Basis	3,296		3,461
Carrying Amount	3,296	[1]	3,461	[1]
Net WAC	6.96%	[2]	6.98%	[2]
Average Yield	6.62%	[2]	7.03%	[2]
ARMs [Member] | Agency Securities [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	10,401,439		7,660,269
Investment Premiums	284,830		158,260
Basis	10,686,269		7,818,529
Carrying Amount	10,904,149	[1]	7,991,122	[1]
Net WAC	2.90%	[2]	3.09%	[2]
Average Yield	2.11%	[2]	2.49%	[2]
ARMs [Member] | Residential Mortgage Loans [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	6,001		7,235
Investment Premiums	22		24
Basis	6,023		7,259
Carrying Amount	6,023	[1]	7,259	[1]
Net WAC	3.48%	[2]	3.45%	[2]
Average Yield	3.52%	[2]	3.79%	[2]
Ginnie Mae ARMs [Member] | Agency Securities [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	1,264,684		489,579
Investment Premiums	35,256		8,030
Basis	1,299,940		497,609
Carrying Amount	1,318,991	[1]	505,376	[1]
Net WAC	3.12%	[2]	3.12%	[2]
Average Yield	2.57%	[2]	2.57%	[2]
Agency Securities [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	11,670,383		8,154,775
Investment Premiums	320,098		166,304
Basis	11,990,481		8,321,079
Carrying Amount	12,227,420	[1]	8,501,453	[1]
Net WAC	2.92%	[2]	3.09%	[2]
Average Yield	2.16%	[2]	2.50%	[2]
Residential Mortgage Loans [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	9,292		10,690
Investment Premiums	27		30
Basis	9,319		10,720
Carrying Amount	9,319	[1]	10,720	[1]
Net WAC	4.71%	[2]	4.59%	[2]
Average Yield	4.61%	[2]	4.83%	[2]
Senior Notes [Member]
Schedule of Mortgage Securities and Similar Investments [Line Items]
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]
Schedule of Mortgage Securities and Similar Investments [Line Items]
Principal Balance	3,307		3,461
Investment Premiums	54		57
Basis	3,361		3,518
Carrying Amount	$ 3,361	[1]	$ 3,518	[1]
Net WAC	8.04%	[2]	7.97%	[2]
Average Yield	7.54%	[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 the weighted average interest rate of the mortgage loans underlying the indicated investments net of servicing and other fees and before amortization of investment premiums as of the indicated balance sheet date. Average yield is presented for the quarter then ended, calculated including the amortization of investment premiums and excluding unrealized gains and losses.