DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 12,693,000	$ (5,470,000)	$ 69,547,000	$ (29,331,000)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	4,640,000	607,000	25,301,000	(6,955,000)
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	1,482,000	(6,450,000)	9,232,000	(16,190,000)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	2,994,000	1,005,000	17,032,000	(3,170,000)
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(130,000)	0	(909,000)	0
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	414,000	827,000	5,627,000	827,000
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 3,293,000	$ (1,459,000)	$ 13,264,000	$ (3,843,000)