ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Contract assets:
Unbilled receivables	$ 6,529	$ 3,425	$ 6,284
Contract liabilities:
Customer advances and unearned income	3,836	3,766	4,516
Unearned event income	$ 8,201	$ 3,864	$ 6,157