SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurements Narrative (Details)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Marketable securities, available-for-sale, amortized cost	$ 18,500,000	$ 0
U.S. treasuries	18,500,000	0
Marketable securities, available-for-sale, accumulated unrealized gain	$ 20,000	$ 0
Minimum | Individual cumulative achievement probabilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.021
Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.155
Maximum | Individual cumulative achievement probabilities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.209
Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.271