DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 45,472	$ (82,858)
Eurodollar Future [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	12	(8,217)
Treasury Note Future [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	2,476	(4,339)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	27,123	(60,623)
Payer Swaption [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(26,167)	0
Payer Swaption [Member] | Long [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	40,070	(2,589)
Interest Rate Floor [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	1,384	0
TBA Contracts [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	9,133	(7,090)
TBA Contracts [Member] | Long [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (8,559)	$ 0