INCOME TAXES - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax asset:
Capitalized technology license	$ 553	$ 804
Capitalized research expenditures	4,710	3,719
Accruals and reserves	2,890	962
Retirement plan	9,570	7,125
Deferred revenue	12,028	206
Tax credit carry forwards	2,895	21,562
Stock-based compensation	1,701	1,819
Other	47	59
Deferred Tax Assets, Gross	34,394	36,256
Valuation allowance	(2,893)	(2,460)
Deferred tax assets	31,501	33,796
Deferred tax liability:
Accruals and reserves	(7,124)	(6,774)
Deferred tax liabilities	(7,124)	(6,774)
Net deferred tax assets	$ 24,377	$ 27,022