FAIR VALUE MEASUREMENTS - Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financial assets
Cash equivalents	$ 53,966	$ 54,998
Marketable debt securities	1,727,451	1,603,615
Marketable equity securities	31,997	66,972
Recurring
Financial assets
Marketable equity securities	31,997	66,972
Total financial assets	1,920,912	1,927,651
Financial liabilities
Development derivative liability	105,233	83,618
Recurring \| Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable equity securities	31,997	66,972
Total financial assets	193,461	324,036
Financial liabilities
Development derivative liability	0	0
Recurring \| Significant Observable Inputs (Level 2)
Financial assets
Marketable equity securities	0	0
Total financial assets	1,727,451	1,603,615
Financial liabilities
Development derivative liability	0	0
Recurring \| Significant Unobservable Inputs (Level 3)
Financial assets
Marketable equity securities	0	0
Total financial assets	0	0
Financial liabilities
Development derivative liability	105,233	83,618
Recurring \| Money market funds
Financial assets
Cash equivalents	160,269	255,869
Restricted cash (money market funds)	1,195	1,195
Recurring \| Money market funds \| Quoted Prices in Active Markets (Level 1)
Financial assets
Cash equivalents	160,269	255,869
Restricted cash (money market funds)	1,195	1,195
Recurring \| Money market funds \| Significant Observable Inputs (Level 2)
Financial assets
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring \| Money market funds \| Significant Unobservable Inputs (Level 3)
Financial assets
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring \| Commercial paper
Financial assets
Cash equivalents	4,999
Marketable debt securities	60,070	78,543
Recurring \| Commercial paper \| Quoted Prices in Active Markets (Level 1)
Financial assets
Cash equivalents	0
Marketable debt securities	0	0
Recurring \| Commercial paper \| Significant Observable Inputs (Level 2)
Financial assets
Cash equivalents	4,999
Marketable debt securities	60,070	78,543
Recurring \| Commercial paper \| Significant Unobservable Inputs (Level 3)
Financial assets
Cash equivalents	0
Marketable debt securities	0	0
Recurring \| U.S. treasury securities
Financial assets
Cash equivalents	48,967	54,998
Marketable debt securities	1,187,418	1,030,578
Recurring \| U.S. treasury securities \| Quoted Prices in Active Markets (Level 1)
Financial assets
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring \| U.S. treasury securities \| Significant Observable Inputs (Level 2)
Financial assets
Cash equivalents	48,967	54,998
Marketable debt securities	1,187,418	1,030,578
Recurring \| U.S. treasury securities \| Significant Unobservable Inputs (Level 3)
Financial assets
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring \| Corporate notes
Financial assets
Marketable debt securities	229,172	253,239
Recurring \| Corporate notes \| Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable debt securities	0	0
Recurring \| Corporate notes \| Significant Observable Inputs (Level 2)
Financial assets
Marketable debt securities	229,172	253,239
Recurring \| Corporate notes \| Significant Unobservable Inputs (Level 3)
Financial assets
Marketable debt securities	0	0
Recurring \| U.S. government-sponsored enterprise securities
Financial assets
Marketable debt securities	188,320	177,741
Recurring \| U.S. government-sponsored enterprise securities \| Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable debt securities	0	0
Recurring \| U.S. government-sponsored enterprise securities \| Significant Observable Inputs (Level 2)
Financial assets
Marketable debt securities	188,320	177,741
Recurring \| U.S. government-sponsored enterprise securities \| Significant Unobservable Inputs (Level 3)
Financial assets
Marketable debt securities	0	0
Recurring \| Certificates of deposit
Financial assets
Marketable debt securities	7,500	7,501
Recurring \| Certificates of deposit \| Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable debt securities	0	0
Recurring \| Certificates of deposit \| Significant Observable Inputs (Level 2)
Financial assets
Marketable debt securities	7,500	7,501
Recurring \| Certificates of deposit \| Significant Unobservable Inputs (Level 3)
Financial assets
Marketable debt securities	0	0
Recurring \| Municipal securities
Financial assets
Marketable debt securities	1,005	1,015
Recurring \| Municipal securities \| Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable debt securities	0	0
Recurring \| Municipal securities \| Significant Observable Inputs (Level 2)
Financial assets
Marketable debt securities	1,005	1,015
Recurring \| Municipal securities \| Significant Unobservable Inputs (Level 3)
Financial assets
Marketable debt securities	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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