DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 4,079,000	$ (8,648,000)	$ (87,630,000)	$ (61,968,000)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(6,000)	(94,000)	(8,324,000)	(14,423,000)
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(113,000)	(1,112,000)	(4,837,000)	(6,311,000)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	489,000	(9,918,000)	(67,713,000)	(36,322,000)
Payer Swaption [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(672,000)	0	(1,561,000)	0
Payer Swaption [Member] | Long [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	914,000	(316,000)	(3,287,000)	(1,379,000)
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	3,431,000	2,479,000	(1,813,000)	(3,846,000)
US Treasury Securities [Member] | Short [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 36,000	$ 0	$ (95,000)	$ 0