DERIVATIVE INSTRUMENTS - Summary of Swaptions Outstanding (Details) - Interest rate swaptions - Long - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Current Notional	$ 4,500,000	$ 6,000,000
Weighted Average Underlying Pay Rate	3.18%	2.62%
Weighted Average Underlying Years to Maturity	10 years 2 months 16 days	9 years 11 months 19 days
Weighted Average Months to Expiration	5 years 33 days	4 months 15 days