RESIDENTIAL MORTGAGE INVESTMENTS - Schedule of Residential Mortgage Investments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	$ 13,450,071		$ 13,222,629
Investment Premiums	441,175		432,744
Amortized Cost Basis	13,891,246		13,655,373
Carrying Amount	14,150,084	[1]	13,908,104	[1]
Net WAC (in hundredths)	2.53%	[2]	2.54%	[2]
Average Yield (in hundredths)	1.70%	[2]	1.66%	[2]
Agency Securities [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	13,443,518		13,215,738
Investment Premiums	441,130		432,696
Amortized Cost Basis	13,884,648		13,648,434
Carrying Amount	14,143,486	[1]	13,901,165	[1]
Net WAC (in hundredths)	2.53%	[2]	2.54%	[2]
Average Yield (in hundredths)	1.69%	[2]	1.66%	[2]
Agency Securities [Member] | Fixed-Rate [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	1,335		1,660
Investment Premiums	4		4
Amortized Cost Basis	1,339		1,664
Carrying Amount	1,340	[1]	1,665	[1]
Net WAC (in hundredths)	6.64%	[2]	6.63%	[2]
Average Yield (in hundredths)	6.02%	[2]	6.52%	[2]
Agency Securities [Member] | ARMs [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	10,381,104		10,230,419
Investment Premiums	334,577		328,781
Amortized Cost Basis	10,715,681		10,559,200
Carrying Amount	10,958,273	[1]	10,800,332	[1]
Net WAC (in hundredths)	2.51%	[2]	2.51%	[2]
Average Yield (in hundredths)	1.76%	[2]	1.68%	[2]
Agency Securities [Member] | Ginnie Mae ARMs [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	3,061,079		2,983,659
Investment Premiums	106,549		103,911
Amortized Cost Basis	3,167,628		3,087,570
Carrying Amount	3,183,873	[1]	3,099,168	[1]
Net WAC (in hundredths)	2.60%	[2]	2.63%	[2]
Average Yield (in hundredths)	1.45%	[2]	1.58%	[2]
Residential Mortgage Loans [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	4,592		4,894
Investment Premiums	13		15
Amortized Cost Basis	4,605		4,909
Carrying Amount	4,605	[1]	4,909	[1]
Net WAC (in hundredths)	4.91%	[2]	4.95%	[2]
Average Yield (in hundredths)	4.12%	[2]	4.09%	[2]
Residential Mortgage Loans [Member] | Fixed-Rate [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	1,701		1,848
Investment Premiums	1		2
Amortized Cost Basis	1,702		1,850
Carrying Amount	1,702	[1]	1,850	[1]
Net WAC (in hundredths)	6.92%	[2]	6.96%	[2]
Average Yield (in hundredths)	5.67%	[2]	5.65%	[2]
Residential Mortgage Loans [Member] | ARMs [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	2,891		3,046
Investment Premiums	12		13
Amortized Cost Basis	2,903		3,059
Carrying Amount	2,903	[1]	3,059	[1]
Net WAC (in hundredths)	3.73%	[2]	3.73%	[2]
Average Yield (in hundredths)	3.22%	[2]	3.18%	[2]
Collateral For Structured Financings [Member]
Schedule of Residential Mortgage Investments [Line Items]
Unpaid Principal Balance	1,961		1,997
Investment Premiums	32		33
Amortized Cost Basis	1,993		2,030
Carrying Amount	$ 1,993	[1]	$ 2,030	[1]
Net WAC (in hundredths)	8.11%	[2]	8.11%	[2]
Average Yield (in hundredths)	7.75%	[2]	7.86%	[2]

[1] Includes unrealized gains and losses for residential mortgage investments classified as available-for-sale (see NOTE 9).

[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.