Residential Mortgage Investments (Schedule Of Residential Mortgage Investments ) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	$ 12,382,603		$ 11,706,960
Investment Premiums	354,545		323,247
Amortized Cost Basis	12,737,148		12,030,207
Carrying Amount	13,012,459	[1]	12,264,906	[1]
Net WAC	2.84%	[2]	2.87%	[2]
Average Yield	2.14%	[2]	2.11%	[2]
Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	12,370,609		11,694,567
Investment Premiums	354,467		323,166
Amortized Cost Basis	12,725,076		12,017,733
Carrying Amount	13,000,387	[1]	12,252,432	[1]
Net WAC	2.84%	[2]	2.87%	[2]
Average Yield	2.14%	[2]	2.11%	[2]
Residential Mortgage Loans [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	8,926		9,121
Investment Premiums	26		27
Amortized Cost Basis	8,952		9,148
Carrying Amount	8,952	[1]	9,148	[1]
Net WAC	4.72%	[2]	4.71%	[2]
Average Yield	4.72%	[2]	4.69%	[2]
Collateral For Structured Financings [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,068		3,272
Investment Premiums	52		54
Amortized Cost Basis	3,120		3,326
Carrying Amount	3,120	[1]	3,326	[1]
Net WAC	8.07%	[2]	8.04%	[2]
Average Yield	7.39%	[2]	7.60%	[2]
Fixed-Rate [Member] | Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,754		4,015
Investment Premiums	10		12
Amortized Cost Basis	3,764		4,027
Carrying Amount	3,771	[1]	4,035	[1]
Net WAC	6.71%	[2]	6.73%	[2]
Average Yield	6.49%	[2]	6.55%	[2]
Fixed-Rate [Member] | Residential Mortgage Loans [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,179		3,234
Investment Premiums	5		5
Amortized Cost Basis	3,184		3,239
Carrying Amount	3,184	[1]	3,239	[1]
Net WAC	6.96%	[2]	6.96%	[2]
Average Yield	6.79%	[2]	6.58%	[2]
ARMs [Member] | Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	11,082,398		10,378,503
Investment Premiums	318,294		285,963
Amortized Cost Basis	11,400,692		10,664,466
Carrying Amount	11,655,508	[1]	10,880,200	[1]
Net WAC	2.83%	[2]	2.85%	[2]
Average Yield	2.11%	[2]	2.08%	[2]
ARMs [Member] | Residential Mortgage Loans [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	5,747		5,887
Investment Premiums	21		22
Amortized Cost Basis	5,768		5,909
Carrying Amount	5,768	[1]	5,909	[1]
Net WAC	3.47%	[2]	3.48%	[2]
Average Yield	3.59%	[2]	3.66%	[2]
Ginnie Mae ARMs [Member] | Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	1,284,457		1,312,049
Investment Premiums	36,163		37,191
Amortized Cost Basis	1,320,620		1,349,240
Carrying Amount	$ 1,341,108	[1]	$ 1,368,197	[1]
Net WAC	2.94%	[2]	3.02%	[2]
Average Yield	2.32%	[2]	2.31%	[2]

[1] Includes unrealized gains and losses for residential mortgage investments classified as available-for-sale (see NOTE 8).

[2] Net WAC, or weighted average coupon, is the weighted average interest rate of the mortgage loans underlying the indicated investments net of servicing and other fees as of the indicated balance sheet date. Net WAC is expressed as a percentage calculated on an annualized basis on the unpaid principal balances of the mortgage loans underlying these investments. Average yield is presented for the quarter then ended, and is based on the cash component of interest income expressed as a percentage calculated on an annualized basis on average amortized cost basis (the "cash yield") less the effects of amortizing investment premiums. Investment premium amortization is determined using the interest method and incorporates actual and anticipated future mortgage prepayments.