SECURED BORROWINGS - Schedule of Secured Borrowings (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Borrowings outstanding	[1],[2]	$ 12,337,299	$ 12,151,122
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		1.46%	1.04%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,980,202	$ 12,807,567
Accrued interest receivable		30,721	28,240
Borrowings outstanding		$ 12,331,060	$ 12,145,346
Average borrowing rates		1.60%	0.96%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,943,193	$ 12,643,359
Accrued interest receivable		30,646	27,889
Borrowings outstanding		$ 12,296,546	$ 11,991,532
Average borrowing rates		1.60%	0.96%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 35,568	$ 162,551
Accrued interest receivable		75	351
Borrowings outstanding		$ 33,073	$ 152,157
Average borrowing rates		1.53%	0.93%
Agency Securities [Member] | Similar Borrowings [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 1,441	$ 1,657
Borrowings outstanding		$ 1,441	$ 1,657
Average borrowing rates		7.94%	7.98%

[1] Amounts include accrued interest payable of $7.7 million and $7.4 million on borrowings under repurchases arrangements as of December 31, 2017 and December 31, 2016, respectively

[2] Included in gross amounts of recognized liabilities at December 31, 2017 is the fair value of non-exchange traded swap agreements (Counterparty 1) and exchange-traded swap agreements (Counterparty 4), calculated including accrued interest. Included in gross amounts offset in the balance sheet are variation margin amounts associated with exchange-traded swap agreements at December 31, 2017.