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DEBT - Components of Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Apr. 15, 2019	Feb. 28, 2019	Dec. 31, 2018	Sep. 30, 2018	Aug. 14, 2018
Line of Credit Facility
Principal amount	$ 518,605			$ 551,420
Less: Unamortized debt financing costs	(14,054)			(16,272)
Less: Current portion	(70,111)			(66,320)
Long-term debt, net	434,440			468,828
Secured Debt \| $495 Million Credit Facility
Line of Credit Facility
Principal amount	416,925			445,000
Less: Unamortized debt financing costs	(12,501)			(14,423)
Maximum borrowing capacity	495,000	$ 495,000	$ 495,000	495,000	$ 495,000
Secured Debt \| $108 Million Credit Facility
Line of Credit Facility
Principal amount	101,680			106,420
Less: Unamortized debt financing costs	(1,553)			(1,849)
Maximum borrowing capacity	$ 108,000			$ 108,000	$ 108,000	$ 108,000