DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (34,288,000)	$ 14,859,000	$ (53,320,000)	$ 56,853,000
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(4,287,000)	6,121,000	(14,329,000)	20,661,000
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(3,523,000)	928,000	(5,199,000)	7,750,000
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(24,109,000)	3,530,000	(26,404,000)	14,037,000
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	0	(430,000)	0	(779,000)
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(685,000)	3,146,000	(1,063,000)	5,213,000
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (1,684,000)	$ 1,564,000	$ (6,325,000)	$ 9,971,000