LONG-TERM DEBT - Summary of Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 31, 2019	Dec. 31, 2018	Feb. 15, 2015
Debt Instruments
Total long-term debt, including current maturities	$ 88,740		$ 85,300
Less current maturities	(1,647)		0
Total long-term debt	87,093		85,300
Credit Facility
Debt Instruments
Total long-term debt, including current maturities	76,550	$ 225,000	$ 85,300	$ 170,000
Swingline sub-facility
Debt Instruments
Total long-term debt, including current maturities		$ 25,000
Obligation under finance lease
Debt Instruments
Total long-term debt, including current maturities	$ 12,190