DERIVATIVE FINANCIAL INTSTRUMENTS - Summary of Outstanding Swaptions (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Assets, at Fair Value	$ 34,207,000	$ 17,160,000
Payer Swaption [Member] | Less Than Or Equal To One Year [Member]
Derivatives, Fair Value [Line Items]
Swaption Cost	9,770,000	2,367,000
Assets, at Fair Value	$ 7,232,000	$ 3,405,000
Derivative Instruments Average Months To Expiration	6 months 11 days	8 months
Notional Amount	$ 950,000,000	$ 200,000,000
Derivative Average Fixed Interest Rate	3.20%	2.16%
Term (Years)	9 years 3 months 7 days	6 years
Receiver Swaption [Member] | Less Than Or Equal To One Year [Member]
Derivatives, Fair Value [Line Items]
Swaption Cost	$ 760,000
Assets, at Fair Value	$ 640,000
Derivative Instruments Average Months To Expiration	7 months 12 days
Notional Amount	$ 100,000,000
Derivative Average Fixed Interest Rate	2.80%
Term (Years)	5 years