SECURED BORROWINGS (Details) - USD ($) $ in Thousands		3 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount		$ 13,330,000	$ 13,480,000
Average borrowings for the effects of related Derivatives held as cash flow hedges		$ 13,009,877	$ 12,843,636
Average rates adjusted for the effects of related Derivatives held as cash flow hedges		0.69%	0.56%
Quarter-end borrowing rates adjusted for effects of related derivative financial instruments ("Derivatives") held as cash flow hedges (see NOTE 6) [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Average borrowing rates		0.72%
Year-end borrowing rates adjusted for effects of related Derivatives held as cash flow hedges [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Average borrowing rates			0.58%
FHLB advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Receivables and other assets		$ 46,000
Collateral carrying amount for FHLB advances		2,356,083
Accrued interest receivable		6,574
Borrowings outstanding for FHLB advance		$ 2,300,000
Average borrowing rates		0.42%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 10,975,946	$ 13,483,572
Accrued interest receivable		22,583	30,933
Borrowings outstanding		10,452,523	$ 12,806,843
Average borrowing rates			0.38%
Agency Securities [Member] | Borrowings under repurchase arrangements with maturities of 30 days or less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		9,177,537	$ 10,401,080
Accrued interest receivable		18,477	24,045
Borrowings outstanding		$ 8,754,427	$ 9,878,889
Average borrowing rates		0.45%	0.35%
Agency Securities [Member] | Borrowings under repurchase arrangements with maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount			$ 1,205,570
Accrued interest receivable			2,248
Borrowings outstanding			$ 1,150,924
Average borrowing rates			0.35%
Agency Securities [Member] | Borrowings under repurchase arrangements with maturities greater than 90 days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 1,796,490	$ 1,874,892
Accrued interest receivable		4,106	4,640
Borrowings outstanding		$ 1,696,177	$ 1,775,000
Average borrowing rates		0.70%	0.56%
Agency Securities [Member] | Similar Borrowings [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount	[1]	$ 1,919	$ 2,030
Accrued interest receivable	[1]	0	0
Borrowings outstanding	[1]	$ 1,919	$ 2,030
Average borrowing rates	[1]	8.11%	8.11%
Agency Securities [Member] | FHLB advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Total collateral carrying amount		$ 13,332,029
Accrued interest receivable		29,157
Borrowings outstanding		$ 12,752,523
Average borrowing rates		0.48%

[1] The maturity of structured financings is directly affected by prepayments on the related mortgage pass-through securities pledged as collateral. Additionally, these financings are subject to redemption by the residual bondholders.