DERIVATIVE INSTRUMENTS - Summary of Swaptions Outstanding (Details) - Interest rate swaptions - Long - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Current Notional	$ 3,200,000	$ 4,075,000
Weighted Average Underlying Pay Rate	2.87%	3.30%
Weighted Average Underlying Years to Maturity	10 years 6 months 14 days	10 years 29 days
Weighted Average Months to Expiration	8 months 22 days	3 months 2 days