RESIDENTIAL MORTGAGE INVESTMENTS - Schedule of Residential Mortgage Investments (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 13,010,978	$ 12,805,510
Investment Premiums		397,876	405,137
Amortized Cost Basis		13,408,854	13,210,647
Carrying Amount	[1]	$ 13,454,098	$ 13,316,282
Net WAC	[2]	2.93%	2.69%
Average Yield	[2]	1.73%	1.56%
Agency Securities [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 13,007,704	$ 12,801,306
Investment Premiums		397,845	405,100
Amortized Cost Basis		13,405,549	13,206,406
Carrying Amount	[1]	$ 13,450,793	$ 13,312,041
Net WAC	[2]	2.93%	2.69%
Average Yield	[2]	1.73%	1.56%
Agency Securities [Member] | Fannie Mae/Freddie Mac [Member] | Fixed-Rate [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 265	$ 385
Investment Premiums		1	1
Amortized Cost Basis		266	386
Carrying Amount	[1]	$ 266	$ 386
Net WAC	[2]	6.51%	6.65%
Average Yield	[2]	6.31%	6.07%
Agency Securities [Member] | Fannie Mae/Freddie Mac [Member] | ARMs [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 10,216,099	$ 10,057,761
Investment Premiums		313,547	314,799
Amortized Cost Basis		10,529,646	10,372,560
Carrying Amount	[1]	$ 10,578,364	$ 10,483,367
Net WAC	[2]	2.97%	2.74%
Average Yield	[2]	1.78%	1.64%
Agency Securities [Member] | Ginnie Mae [Member] | ARMs [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 2,791,340	$ 2,743,160
Investment Premiums		84,297	90,300
Amortized Cost Basis		2,875,637	2,833,460
Carrying Amount	[1]	$ 2,872,163	$ 2,828,288
Net WAC	[2]	2.78%	2.51%
Average Yield	[2]	1.54%	1.29%
Residential Mortgage Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 1,856	$ 2,574
Investment Premiums		8	10
Amortized Cost Basis		1,864	2,584
Carrying Amount	[1]	$ 1,864	$ 2,584
Net WAC	[2]	4.98%	4.63%
Average Yield	[2]	3.59%	3.24%
Residential Mortgage Loans [Member] | Fixed-Rate [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 645	$ 735
Investment Premiums		1	1
Amortized Cost Basis		646	736
Carrying Amount	[1]	$ 646	$ 736
Net WAC	[2]	6.74%	6.72%
Average Yield	[2]	4.48%	3.87%
Residential Mortgage Loans [Member] | ARMs [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 1,211	$ 1,839
Investment Premiums		7	9
Amortized Cost Basis		1,218	1,848
Carrying Amount	[1]	$ 1,218	$ 1,848
Net WAC	[2]	4.04%	3.80%
Average Yield	[2]	3.20%	3.00%
Collateral for Structured Financings [Member]
Financing Receivable, Recorded Investment [Line Items]
Unpaid Principal Balance		$ 1,418	$ 1,630
Investment Premiums		23	27
Amortized Cost Basis		1,441	1,657
Carrying Amount	[1]	$ 1,441	$ 1,657
Net WAC	[2]	7.94%	7.98%
Average Yield	[2]	7.81%	7.78%

[1] Includes unrealized gains and losses for residential mortgage investments classified as available-for-sale.

[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 year 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.