DERIVATIVE FINANCIAL INSTRUMENTS - Effect on the consolidated statements of operations (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 41,994,000	$ (4,419,000)
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	14,541,000	(571,000)
Treasury Note Future [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	6,821,000	(3,851,000)
Swap [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	10,508,000	3,000
Receiver Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	(349,000)	0
Payer Swaption [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	2,066,000	0
TBA Contracts [Member]
Derivatives, Fair Value [Line Items]
(Losses) gains on derivative instruments	$ 8,407,000	$ 0