RESIDENTIAL MORTGAGE INVESTMENTS - Schedule of Residential Mortgage Investments (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	$ 13,090,213		$ 13,128,734
Investment Premiums	425,686		404,975
Amortized Cost Basis	13,515,899		13,533,709
Carrying Amount	13,738,311	[1]	13,860,158	[1]
Net WAC (in hundredths)	2.62%	[2]	2.71%	[2]
Average Yield (in hundredths)	1.36%	[2]	1.76%	[2]
Agency Securities [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	13,080,734		13,117,897
Investment Premiums	425,628		404,903
Amortized Cost Basis	13,506,362		13,522,800
Carrying Amount	13,728,774	[1]	13,849,249	[1]
Net WAC (in hundredths)	2.62%	[2]	2.70%	[2]
Average Yield (in hundredths)	1.36%	[2]	1.75%	[2]
Agency Securities [Member] | Fixed-Rate [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,307		3,194
Investment Premiums	6		9
Amortized Cost Basis	2,313		3,203
Carrying Amount	2,316	[1]	3,208	[1]
Net WAC (in hundredths)	6.68%	[2]	6.70%	[2]
Average Yield (in hundredths)	6.41%	[2]	6.47%	[2]
Agency Securities [Member] | ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	11,020,341		11,547,954
Investment Premiums	354,718		356,646
Amortized Cost Basis	11,375,059		11,904,600
Carrying Amount	11,586,806	[1]	12,198,922	[1]
Net WAC (in hundredths)	2.61%	[2]	2.69%	[2]
Average Yield (in hundredths)	1.38%	[2]	1.72%	[2]
Agency Securities [Member] | Ginnie Mae ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,058,086		1,566,749
Investment Premiums	70,904		48,248
Amortized Cost Basis	2,128,990		1,614,997
Carrying Amount	2,139,652	[1]	1,647,119	[1]
Net WAC (in hundredths)	2.64%	[2]	2.77%	[2]
Average Yield (in hundredths)	1.19%	[2]	1.95%	[2]
Residential Mortgage Loans [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	7,221		8,038
Investment Premiums	21		25
Amortized Cost Basis	7,242		8,063
Carrying Amount	7,242	[1]	8,063	[1]
Net WAC (in hundredths)	5.02%	[2]	5.04%	[2]
Average Yield (in hundredths)	4.28%	[2]	4.71%	[2]
Residential Mortgage Loans [Member] | Fixed-Rate [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,693		3,007
Investment Premiums	3		5
Amortized Cost Basis	2,696		3,012
Carrying Amount	2,696	[1]	3,012	[1]
Net WAC (in hundredths)	7.00%	[2]	7.01%	[2]
Average Yield (in hundredths)	5.67%	[2]	6.15%	[2]
Residential Mortgage Loans [Member] | ARMs [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	4,528		5,031
Investment Premiums	18		20
Amortized Cost Basis	4,546		5,051
Carrying Amount	4,546	[1]	5,051	[1]
Net WAC (in hundredths)	3.85%	[2]	3.87%	[2]
Average Yield (in hundredths)	3.46%	[2]	3.85%	[2]
Collateral For Structured Financings [Member]
Schedule Of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,258		2,799
Investment Premiums	37		47
Amortized Cost Basis	2,295		2,846
Carrying Amount	$ 2,295	[1]	$ 2,846	[1]
Net WAC (in hundredths)	8.08%	[2]	8.12%	[2]
Average Yield (in hundredths)	7.57%	[2]	7.57%	[2]

[1] Includes unrealized gains and losses for residential mortgage investments classified as available-for-sale (see NOTE 10).

[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.