FAIR VALUE MEASUREMENTS - Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 136,492	$ 340,132
Marketable debt securities	972,838	1,315,149
Marketable equity securities	38,078	13,967
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	38,078	13,967
Total	1,267,026	1,450,480
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	38,078	13,967
Total	294,188	135,331
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	0	0
Total	972,838	1,315,149
Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	0	0
Total	0	0
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		3,439
Marketable debt securities	21,792	36,474
Recurring | Commercial paper | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Marketable debt securities	0	0
Recurring | Commercial paper | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		3,439
Marketable debt securities	21,792	36,474
Recurring | Commercial paper | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
Marketable debt securities	0	0
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	136,492	336,693
Marketable debt securities	589,843	755,714
Recurring | U.S. treasury securities | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | U.S. treasury securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	136,492	336,693
Marketable debt securities	589,843	755,714
Recurring | U.S. treasury securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	254,627	119,882
Restricted cash (money market funds)	1,483	1,482
Recurring | Money market funds | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	254,627	119,882
Restricted cash (money market funds)	1,483	1,482
Recurring | Money market funds | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Money market funds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	5,134	4,301
Recurring | Certificates of deposit | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Certificates of deposit | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	5,134	4,301
Recurring | Certificates of deposit | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	114,280	146,040
Recurring | Corporate notes | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Corporate notes | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	114,280	146,040
Recurring | Corporate notes | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	105,297	32,488
Recurring | U.S. government-sponsored enterprise securities | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | U.S. government-sponsored enterprise securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	105,297	32,488
Recurring | U.S. government-sponsored enterprise securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	$ 0	$ 0