FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value on a Recurring Basis
Total	$ 270,428	$ 156,382
Money market funds
Fair Value on a Recurring Basis
Total	41,608	6,671
U.S. Treasury notes
Fair Value on a Recurring Basis
Total	5,636
Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	14,000	13,512
Commercial paper
Fair Value on a Recurring Basis
Total	88,094	44,130
Corporate notes
Fair Value on a Recurring Basis
Total	120,705	91,680
Equity Investment
Fair Value on a Recurring Basis
Total	385	389
Level 1
Fair Value on a Recurring Basis
Total	41,608	6,671
Level 1 | Money market funds
Fair Value on a Recurring Basis
Total	41,608	6,671
Level 2
Fair Value on a Recurring Basis
Total	228,820	149,711
Level 2 | U.S. Treasury notes
Fair Value on a Recurring Basis
Total	5,636
Level 2 | Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	14,000	13,512
Level 2 | Commercial paper
Fair Value on a Recurring Basis
Total	88,094	44,130
Level 2 | Corporate notes
Fair Value on a Recurring Basis
Total	120,705	91,680
Level 2 | Equity Investment
Fair Value on a Recurring Basis
Total	$ 385	$ 389