FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Financial assets and liabilities by hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 18,574	$ 37,172
Level 2:
Amortized Cost	336,119
Debt Securities, Available-for-sale	336,404
Total assets measured at fair value
Short-term investments	289,424	203,906
Long-term investments	46,980	2,287
Fair Value, Inputs, Level 2
Level 2:
Gross Unrealized Losses	(36)	(160)
Fair Value, Inputs, Level 2 | U.S. government agency securities
Level 2:
Gross Unrealized Losses	(14)	0
Fair Value, Inputs, Level 2 | Commercial paper
Level 2:
Gross Unrealized Losses	(2)	(63)
Fair Value, Inputs, Level 2 | Corporate debt securities
Level 2:
Gross Unrealized Losses	(20)	(63)
Fair Value, Inputs, Level 2 | Asset-backed securities
Level 2:
Gross Unrealized Losses	0	(34)
Fair Value, Measurements, Recurring
Total assets measured at fair value
Amortized Cost	354,693	243,518
Gross Unrealized Gains	321	7
Gross Unrealized Losses	(36)	(160)
Fair Value	354,978	243,365
Fair Value, Measurements, Recurring | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	18,574	37,172
Fair Value, Measurements, Recurring | Short-term Investments
Total assets measured at fair value
Short-term investments	289,424	203,906
Fair Value, Measurements, Recurring | Long-term investments
Total assets measured at fair value
Long-term investments	46,980	2,287
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		33,154
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Level 2:
Amortized Cost	336,119	207,343
Gross Unrealized Gains	321	7
Gross Unrealized Losses	(36)	(160)
Debt Securities, Available-for-sale	336,404	207,190
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Level 2:
Debt Securities, Available-for-sale	0	997
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Short-term Investments
Total assets measured at fair value
Short-term investments	289,424	203,906
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Long-term investments
Total assets measured at fair value
Long-term investments	46,980	2,287
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities
Level 2:
Amortized Cost	40,741
Gross Unrealized Gains	6
Gross Unrealized Losses	(14)
Debt Securities, Available-for-sale	40,733
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities | Cash and Cash Equivalents
Level 2:
Debt Securities, Available-for-sale	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities | Short-term Investments
Total assets measured at fair value
Short-term investments	19,990
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities | Long-term investments
Total assets measured at fair value
Long-term investments	20,743
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial paper
Level 2:
Amortized Cost	108,248	68,467
Gross Unrealized Gains	107	0
Gross Unrealized Losses	(2)	(63)
Debt Securities, Available-for-sale	108,353	68,404
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial paper | Cash and Cash Equivalents
Level 2:
Debt Securities, Available-for-sale	0	997
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial paper | Short-term Investments
Total assets measured at fair value
Short-term investments	108,353	67,407
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Commercial paper | Long-term investments
Total assets measured at fair value
Long-term investments	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities
Level 2:
Amortized Cost	185,569	89,038
Gross Unrealized Gains	205	4
Gross Unrealized Losses	(20)	(63)
Debt Securities, Available-for-sale	185,754	88,979
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities | Cash and Cash Equivalents
Level 2:
Debt Securities, Available-for-sale	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities | Short-term Investments
Total assets measured at fair value
Short-term investments	159,517	86,692
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities | Long-term investments
Total assets measured at fair value
Long-term investments	26,237	2,287
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset-backed securities
Level 2:
Amortized Cost	1,561	49,838
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	(34)
Debt Securities, Available-for-sale	1,564	49,807
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset-backed securities | Cash and Cash Equivalents
Level 2:
Debt Securities, Available-for-sale	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset-backed securities | Short-term Investments
Total assets measured at fair value
Short-term investments	1,564	49,807
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Asset-backed securities | Long-term investments
Total assets measured at fair value
Long-term investments	0	0
Cash | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,393	3,021
Cash and Cash Equivalents, Fair Value Disclosure	1,393	3,021
Cash | Fair Value, Measurements, Recurring | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,393	3,021
Money market fund | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	17,181	33,154
Cash and Cash Equivalents, Fair Value Disclosure	17,181	$ 33,154
Money market fund | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 17,181