DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (51,176,000)	$ 24,311,000	$ (15,349,000)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(13,860,000)	7,170,000	1,257,000
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(5,175,000)	5,507,000	(14,922,000)
Fed Funds Futures [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	177,000	0	0
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(26,582,000)	8,609,000	3,216,000
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	0	105,000	0
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(1,379,000)	(1,607,000)	1,038,000
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (4,357,000)	$ 4,527,000	$ (5,938,000)