FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash equivalents:
Cash equivalents		$ 197,615
Short-term investments:
Short-term investments	$ 11,874	0
Total assets	20,602
Time deposits
Short-term investments:
Short-term investments	11,874
Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Cash equivalents		147,615
Short-term investments:
Total assets	1,780
Quoted Market Prices in Active Markets for Identical Assets (Level 1) | Time deposits
Short-term investments:
Short-term investments	0
Significant Other Observable Inputs (Level 2)
Cash equivalents:
Cash equivalents		50,000
Short-term investments:
Total assets	18,822
Significant Other Observable Inputs (Level 2) | Time deposits
Short-term investments:
Short-term investments	11,874
Money market funds
Cash equivalents:
Cash equivalents	1,780	147,615
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Cash equivalents	1,780	147,615
Money market funds | Significant Other Observable Inputs (Level 2)
Cash equivalents:
Cash equivalents	0	0
Time deposits
Cash equivalents:
Cash equivalents	6,948	50,000
Time deposits | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Cash equivalents	0	0
Time deposits | Significant Other Observable Inputs (Level 2)
Cash equivalents:
Cash equivalents	$ 6,948	$ 50,000