SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract assets (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unbilled Receivables Current [Roll Forward]
Unbilled receivables, Balance at Beginning of Period	$ 1,429	$ 2,259
Unbilled receivables, Revenue Recognized In Advance of Billings	3,278	198
Unbilled receivables, Billings	(1,428)	(1,028)
Unbilled receivables, Balance at End of Period	$ 3,279	$ 1,429