Residential Mortgage Investments (Schedule Of Residential Mortgage Investment) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	$ 13,529,925		$ 11,706,960
Investment Premiums	414,001		323,247
Amortized Cost Basis	13,943,926		12,030,207
Carrying Amount	14,313,208	[1]	12,264,906	[1]
Net WAC	2.75%	[2]	2.87%	[2]
Average Yield	1.86%	[2]	2.11%	[2]
Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	13,518,891		11,694,567
Investment Premiums	413,928		323,166
Amortized Cost Basis	13,932,819		12,017,733
Carrying Amount	14,302,101	[1]	12,252,432	[1]
Net WAC	2.74%	[2]	2.87%	[2]
Average Yield	1.86%	[2]	2.11%	[2]
Agency Securities [Member] | Fixed-Rate [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,417		4,015
Investment Premiums	10		12
Amortized Cost Basis	3,427		4,027
Carrying Amount	3,433	[1]	4,035	[1]
Net WAC	6.70%	[2]	6.73%	[2]
Average Yield	6.48%	[2]	6.55%	[2]
Agency Securities [Member] | ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	11,887,228		10,378,503
Investment Premiums	363,586		285,963
Amortized Cost Basis	12,250,814		10,664,466
Carrying Amount	12,586,241	[1]	10,880,200	[1]
Net WAC	2.74%	[2]	2.85%	[2]
Average Yield	1.82%	[2]	2.08%	[2]
Agency Securities [Member] | Ginnie Mae ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	1,628,246		1,312,049
Investment Premiums	50,332		37,191
Amortized Cost Basis	1,678,578		1,349,240
Carrying Amount	1,712,427	[1]	1,368,197	[1]
Net WAC	2.78%	[2]	3.02%	[2]
Average Yield	2.10%	[2]	2.31%	[2]
Residential Mortgage Loans [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	8,205		9,121
Investment Premiums	25		27
Amortized Cost Basis	8,230		9,148
Carrying Amount	8,230	[1]	9,148	[1]
Net WAC	4.80%	[2]	4.71%	[2]
Average Yield	4.69%	[2]	4.69%	[2]
Residential Mortgage Loans [Member] | Fixed-Rate [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,066		3,234
Investment Premiums	5		5
Amortized Cost Basis	3,071		3,239
Carrying Amount	3,071	[1]	3,239	[1]
Net WAC	6.96%	[2]	6.96%	[2]
Average Yield	6.15%	[2]	6.58%	[2]
Residential Mortgage Loans [Member] | ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	5,139		5,887
Investment Premiums	20		22
Amortized Cost Basis	5,159		5,909
Carrying Amount	5,159	[1]	5,909	[1]
Net WAC	3.51%	[2]	3.48%	[2]
Average Yield	3.83%	[2]	3.66%	[2]
Collateral For Structured Financings [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,829		3,272
Investment Premiums	48		54
Amortized Cost Basis	2,877		3,326
Carrying Amount	$ 2,877	[1]	$ 3,326	[1]
Net WAC	8.12%	[2]	8.04%	[2]
Average Yield	7.39%	[2]	7.60%	[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.