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INVESTMENTS AND FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities $ 18,806  
Funds Held for Clients    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 18,806 $ 4,256
Money market funds 23,508 0
Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 11,588 $ 8,111