DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 14,859,000	$ (19,442,000)	$ 56,853,000	$ (23,861,000)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	6,121,000	(6,990,000)	20,661,000	(7,562,000)
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	928,000	(5,890,000)	7,750,000	(9,740,000)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	3,530,000	(4,178,000)	14,037,000	(4,175,000)
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(430,000)	0	(779,000)	0
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	3,146,000	0	5,213,000	0
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 1,564,000	$ (2,384,000)	$ 9,971,000	$ (2,384,000)