NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rollforward of the allowance for doubtful accounts
Balance at beginning of period	$ 2,590	$ 2,124	$ 4,495
Charged to expense	5,814	2,837	1,974
Net write-offs	(6,025)	(2,371)	(4,345)
Balance at end of period	$ 2,379	$ 2,590	$ 2,124