FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Available-for-sale investments in continuous unrealized loss position not deemed to be OTTI (Details) - Level 2 - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 103,083	$ 68,329
Unrealized Losses	(383)	(36)
U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	24,235
Unrealized Losses	0	(14)
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	5,426
Unrealized Losses	0	(2)
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	103,083	38,668
Unrealized Losses	$ (383)	$ (20)