FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Marketable securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	$ 10,555	$ 18,574
Level 2:
Short-term investments	273,738	289,424
Long-term investments	19,954	46,980
Fair Value, Measurements, Recurring
Total assets measured at fair value
Amortized Cost	304,194	354,693
Gross Unrealized Gains	436	321
Gross Unrealized Losses	383	36
Estimated Fair Value	304,247	354,978
Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	2,257	1,393
Level 1:
Estimated Fair Value	2,257	1,393
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	10,555	18,574
Cash and Cash Equivalents | Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	2,257	1,393
Short-Term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	273,738	289,424
Long-Term Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	19,954	46,980
Level 1: | Money market fund | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	8,298	17,181
Level 1:
Estimated Fair Value	8,298	17,181
Level 1: | Cash and Cash Equivalents | Money market fund | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash and Cash Equivalents	8,298	17,181
Level 2:
Level 2:
Gross Unrealized Losses	(383)	(36)
Level 2: | U.S. government agency securities
Level 2:
Gross Unrealized Losses	0	(14)
Level 2: | Commercial paper
Level 2:
Gross Unrealized Losses	0	(2)
Level 2: | Corporate debt securities
Level 2:
Gross Unrealized Losses	(383)	(20)
Level 2: | Fair Value, Measurements, Recurring
Level 2:
Amortized Cost	293,639	336,119
Gross Unrealized Gains	436	321
Gross Unrealized Losses	(383)	(36)
Estimated Fair Value	293,692	336,404
Level 2: | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Amortized Cost	24,247	40,741
Gross Unrealized Gains	13	6
Gross Unrealized Losses	0	(14)
Estimated Fair Value	24,260	40,733
Level 2: | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Amortized Cost	83,255	108,248
Gross Unrealized Gains	300	107
Gross Unrealized Losses	0	(2)
Estimated Fair Value	83,555	108,353
Level 2: | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Amortized Cost	186,137	185,569
Gross Unrealized Gains	123	205
Gross Unrealized Losses	(383)	(20)
Estimated Fair Value	185,877	185,754
Level 2: | Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Level 2:
Amortized Cost		1,561
Gross Unrealized Gains		3
Gross Unrealized Losses		0
Estimated Fair Value		1,564
Level 2: | Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Level 2:
Estimated Fair Value	0	0
Level 2: | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Estimated Fair Value	0	0
Level 2: | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Estimated Fair Value	0	0
Level 2: | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Estimated Fair Value	0	0
Level 2: | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Level 2:
Estimated Fair Value		0
Level 2: | Short-Term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	273,738	289,424
Level 2: | Short-Term Investments | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Short-term investments	21,505	19,990
Level 2: | Short-Term Investments | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Short-term investments	83,555	108,353
Level 2: | Short-Term Investments | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Short-term investments	168,678	159,517
Level 2: | Short-Term Investments | Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Level 2:
Short-term investments		1,564
Level 2: | Long-Term Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	19,954	46,980
Level 2: | Long-Term Investments | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Long-term investments	2,755	20,743
Level 2: | Long-Term Investments | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Long-term investments	0	0
Level 2: | Long-Term Investments | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Long-term investments	$ 17,199	26,237
Level 2: | Long-Term Investments | Fair Value, Measurements, Recurring | Asset-backed Securities [Member]
Level 2:
Long-term investments		$ 0