ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Contract assets:
Unbilled receivables	$ 8,540	$ 5,798	$ 3,892
Contract liabilities:
Customer advances and unearned income	5,252	4,955	2,918
Unearned event income	$ 6,118	$ 5,921	$ 10,352