SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract liabilities (Details 4) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Movement in Deferred Revenue [Roll Forward]
Deferred revenue, Balance at Beginning of Period	$ 2,932	$ 2,933
Deferred revenue, Billings	5,564	4,933
Deferred revenue recognized	(5,397)	(4,934)
Deferred revenue, Balance at End of Period	$ 3,099	$ 2,932

Amount of deferred revenue recognized for transactions arising during the current reporting period. Deferred revenue is a liability as of the balance sheet date related to a revenue producing activity for which revenue has not yet been recognized. Generally, an Entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

No definition available.

Name:	us-gaap_DeferredRevenueAdditions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration