SECURED BORROWINGS - Schedule of Secured Borrowings (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,590,000	$ 12,980,000
Borrowings outstanding	[1],[2]	$ 11,953,111	$ 12,337,299
Quarter-End Borrowing Rates Adjusted for Effects of Related Derivative Financial Instruments ("Derivatives") Held as Cash Flow Hedges [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Average borrowing rates		1.63%
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%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,592,953	$ 12,980,202
Accrued interest receivable		31,441	30,721
Borrowings outstanding		$ 11,944,841	$ 12,331,060
Average borrowing rates		1.88%	1.60%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 12,559,176	$ 12,943,193
Accrued interest receivable		31,369	30,646
Borrowings outstanding		$ 11,913,651	$ 12,296,546
Average borrowing rates		1.88%	1.60%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 32,595	$ 35,568
Accrued interest receivable		72	75
Borrowings outstanding		$ 30,008	$ 33,073
Average borrowing rates		1.78%	1.53%
Agency Securities [Member] | Similar Borrowings [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 1,182	$ 1,441
Borrowings outstanding		$ 1,182	$ 1,441
Average borrowing rates		8.17%	7.94%

[1] Amounts include accrued interest payable of $9.5 million and $7.7 million on borrowings under repurchase arrangements as of March 31, 2018 and December 31, 2017, respectively.

[2] Included in gross amounts of recognized liabilities at March 31, 2018 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 March 31, 2018.