SECURED BORROWINGS - Schedule of Secured Borrowings (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 7,710,000	$ 10,830,000
Borrowings outstanding	[1],[2]	7,320,090	10,286,011
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		7,711,992	10,828,712
Accrued interest receivable		17,989	31,568
Borrowings outstanding		$ 7,319,083	$ 10,274,498
Average borrowing rates		0.21%	2.10%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 5,249,989	$ 9,484,275
Accrued interest receivable		12,597	27,826
Borrowings outstanding		$ 4,972,181	$ 9,002,527
Average borrowing rates		0.21%	2.12%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 1,939,034	$ 1,344,437
Accrued interest receivable		4,225	3,742
Borrowings outstanding		$ 1,846,902	$ 1,271,971
Average borrowing rates		0.20%	1.98%
Agency Securities [Member] | Borrowings with Maturities Greater than 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Collateral carrying amount		$ 522,969
Accrued interest receivable		1,167
Borrowings outstanding		$ 500,000
Average borrowing rates		0.29%

[1] Amounts include accrued interest payable of $1.0 million and $11.5 million on borrowings under repurchases arrangements as of December 31, 2020 and December 31, 2019, respectively

[2] Included in gross amounts of recognized liabilities at December 31, 2020 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, 2020.