NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Rollforward of the allowance for doubtful accounts
Balance at beginning of period	$ 2,124	$ 4,495	$ 4,246
Charged to expense	2,837	1,974	1,292
Net write-offs	(2,371)	(4,345)	(1,043)
Balance at end of period	$ 2,590	$ 2,124	$ 4,495