DERIVATIVE INSTRUMENTS - Summary of Swaptions Outstanding (Details) - Interest rate swaptions - Long - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Current Notional	$ 2,800,000	$ 4,075,000
Weighted Average Underlying Pay Rate	3.12%	3.30%
Weighted Average Underlying Years to Maturity	10 years 4 months	10 years 1 month
Weighted Average Months to Expiration	6 months 21 days	3 months 2 days