SECURED BORROWINGS - Schedule of Secured Borrowings (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged		$ 7,160,000	$ 7,710,000
Borrowings outstanding	[1],[2]	6,806,138	7,320,090
Borrowings with Maturities Greater than 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged			522,969
Accrued interest receivable			1,167
Borrowings outstanding			$ 500,000
Average borrowing rates			0.29%
Agency Securities [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged		7,159,285	$ 7,711,992
Accrued interest receivable		16,397	17,989
Borrowings outstanding		$ 6,805,061	$ 7,319,083
Average borrowing rates		0.17%	0.21%
Agency Securities [Member] | Borrowings with Maturities of 30 Days or Less [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged		$ 6,420,936	$ 5,249,989
Accrued interest receivable		14,801	12,597
Borrowings outstanding		$ 6,095,509	$ 4,972,181
Average borrowing rates		0.16%	0.21%
Agency Securities [Member] | Borrowings with Maturities of 31 to 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged		$ 217,480	$ 1,939,034
Accrued interest receivable		464	4,225
Borrowings outstanding		$ 209,552	$ 1,846,902
Average borrowing rates		0.16%	0.20%
Agency Securities [Member] | Borrowings with Maturities Greater than 90 Days [Member]
Secured Borrowings, Including Interest Rate Hedging Activity [Line Items]
Agency securities pledged		$ 520,869
Accrued interest receivable		1,132
Borrowings outstanding		$ 500,000
Average borrowing rates		0.29%

[1] Amounts include accrued interest payable of $1.1 million and $1.0 million on borrowings under repurchase arrangements as of March 31, 2021 and December 31, 2020, respectively.

[2] Included in gross amounts of recognized liabilities at March 31, 2021 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, 2021.