SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for doubtful accounts (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for doubtful accounts balance - beginning of year	$ 20	$ 20
Additions to the allowance for doubtful accounts	0	19
Deductions against the allowance for doubtful accounts	0	(19)
Allowance for doubtful accounts balance - end of year	$ 20	$ 20