SECURED BORROWINGS - Schedule of Secured Borrowings (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Borrowings outstanding	[1]	$ 12,151,122	$ 10,090,846
FHLB Advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Accrued interest receivable			$ 11,422
Average borrowing rates			0.43%
Collateral carrying amount for FHLB advances			$ 2,956,908
Borrowings outstanding for FHLB advance			$ 2,875,000
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		1.04%	0.85%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,807,567	$ 10,579,208
Accrued interest receivable		28,240	21,884
Borrowings outstanding		$ 12,145,346	10,083,394
Average borrowing rates		0.96%
Agency Securities [Member] | FHLB Advances [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount			13,536,116
Accrued interest receivable			33,306
Borrowings outstanding			$ 12,958,394
Average borrowing rates			0.62%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,643,359	$ 9,080,363
Accrued interest receivable		27,889	18,504
Borrowings outstanding		$ 11,991,532	$ 8,585,336
Average borrowing rates		0.96%	0.67%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 162,551	$ 423,710
Accrued interest receivable		351	861
Borrowings outstanding		$ 152,157	$ 346,177
Average borrowing rates		0.93%	0.63%
Agency Securities [Member] | Borrowings with Maturities Greater than 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount			$ 1,073,254
Accrued interest receivable			2,519
Borrowings outstanding			$ 1,150,000
Average borrowing rates			0.75%
Agency Securities [Member] | Similar Borrowings [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 1,657	$ 1,881
Borrowings outstanding		$ 1,657	$ 1,881
Average borrowing rates		7.98%	8.12%

[1] Amounts include accrued interest of $12.0 million and $10.9 million on interest rate swap agreements and $7.4 million and $9.3 million on borrowings under repurchase arrangements, included in “Accounts payable and accrued expenses” on the face of the Balance Sheets as of December 31, 2016 and December 31, 2015, respectively.