DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (15,349)	$ (9,387)	$ (22,890)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	1,257	(12,808)	(17,741)
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(14,922)	(3,600)	(3,887)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	3,216	9,503	0
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	0	36	(431)
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	1,038	0	(1,217)
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ (5,938)	$ (2,518)	$ 386