DERIVATIVE FINANCIAL INTSTRUMENTS - Summary of Outstanding Swaptions (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Assets, at Fair Value	$ 36,179,000	$ 17,160,000
Payer Swaption [Member] | Less Than Or Equal To One Year [Member]
Derivatives, Fair Value [Line Items]
Swaption Cost	11,062,000	2,367,000
Assets, at Fair Value	$ 10,296,000	$ 3,405,000
Derivative Instruments Average Months To Expiration	9 months 9 days	8 months
Notional Amount	$ 750,000,000	$ 200,000,000
Derivative Average Fixed Interest Rate	3.00%	2.16%
Term (Years)	9 years 2 months 13 days	6 years
Receiver Swaption [Member] | Less Than Or Equal To One Year [Member]
Derivatives, Fair Value [Line Items]
Swaption Cost	$ 820,000
Assets, at Fair Value	$ 760,000
Derivative Instruments Average Months To Expiration	10 months 12 days
Notional Amount	$ 100,000,000
Derivative Average Fixed Interest Rate	2.60%
Term (Years)	5 years