INCOME TAXES - Reconciliation of Unrecognized Income Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning balance	$ 6,599	$ 3,721	$ 2,798
Additions for tax positions of prior years	576	1,754	338
Additions for tax positions of current years	1,646	1,589	1,094
Expiration of the statute of limitations	(788)	(439)	(366)
Reductions for tax positions of prior years	(13)	(26)	(143)
Unrecognized tax benefits, ending balance	$ 8,020	$ 6,599	$ 3,721