DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (19,032,000)	$ 41,994,000
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(10,041,000)	14,541,000
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(1,677,000)	6,821,000
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(2,295,000)	10,508,000
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	0	(349,000)
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(378,000)	2,066,000
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (4,641,000)	$ 8,407,000