FAIR VALUE MEASUREMENTS - Summary of Fair Value Measurements of Cash Equivalents, Available-for-Sale Marketable Securities and Free Share Asset/Liability (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 197,410	$ 303,878
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	116,393	30,496
Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		2,999
Total marketable securities	90,179	155,368
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	81,844	95,017
U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	25,387	53,493
Fair value on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	116,393	33,495
Total marketable securities	197,410	303,878
Total cash equivalents and marketable securities and free shares asset	313,803	337,373
Fair value on recurring basis | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	116,393	30,496
Fair value on recurring basis | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		2,999
Total marketable securities	90,179	155,368
Fair value on recurring basis | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	81,844	95,017
Fair value on recurring basis | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	25,387	53,493
Fair value on recurring basis | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	167	236
Fair value on recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	116,393	30,496
Total marketable securities	0	0
Total cash equivalents and marketable securities and free shares asset	116,393	30,496
Fair value on recurring basis | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	116,393	30,496
Fair value on recurring basis | Level 1 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		0
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 1 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	0	0
Fair value on recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	2,999
Total marketable securities	197,410	303,878
Total cash equivalents and marketable securities and free shares asset	197,410	306,877
Fair value on recurring basis | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Fair value on recurring basis | Level 2 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		2,999
Total marketable securities	90,179	155,368
Fair value on recurring basis | Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	81,844	95,017
Fair value on recurring basis | Level 2 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	25,387	53,493
Fair value on recurring basis | Level 2 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	0	0
Fair value on recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Total marketable securities	0	0
Total cash equivalents and marketable securities and free shares asset	0	0
Fair value on recurring basis | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Fair value on recurring basis | Level 3 | Commercial paper securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		0
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | U.S. government-sponsored entity debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	0	0
Fair value on recurring basis | Level 3 | Free shares asset
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities and free shares asset	$ 167	$ 236