INCOME TAXES - Summary of Changes in Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits balance, January 1,	$ 187,000	$ 0	$ 408,000
Addition for tax positions taken in a prior year	162,000	187,000	0	$ 0	$ 400,000
Deduction for tax positions taken in a prior year	(187,000)	0	(408,000)
Unrecognized tax benefits balance, December 31,	$ 162,000	$ 187,000	$ 0	$ 408,000