ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Contract assets:
Unbilled receivables	$ 3,530	$ 3,425	$ 7,226
Contract liabilities:
Customer advances and unearned income	3,634	3,766	4,681
Unearned event income	$ 3,848	$ 3,864	$ 2,368