ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract assets and liabilities (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Contract assets:
Unbilled receivables	$ 5,968	$ 4,268	$ 3,425
Contract liabilities:
Customer advances and unearned income	3,403	4,588	3,766
Unearned event income	$ 3,831	$ 3,163	$ 3,864