SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Schedule of Changes in Allowance for Doubtful Accounts) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts:
Balance, beginning of period	$ 4,992	$ 3,208	$ 1,627
Add: bad debt expense	16,883	15,913	10,271
Less: write-offs, net of recoveries	(14,523)	(14,129)	(8,690)
Balance, end of period	$ 7,352	$ 4,992	$ 3,208