Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Unrealized Gains		$ 1,100
Gross Unrealized Losses		(200)
Cash	$ 7,559	6,285
Total cash and investments in marketable securities	1,133,409	1,198,558
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	993,849
Gross Unrealized Gains	352
Gross Unrealized Losses	(212)
Available-for-sale investments	993,989	1,003,348
Corporate notes and bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	510,521
Gross Unrealized Gains	313
Gross Unrealized Losses	(171)
Available-for-sale investments	510,663	687,469
Corporate commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	478,448
Gross Unrealized Gains	35
Gross Unrealized Losses	(41)
Available-for-sale investments	478,442	313,497
Obligations of U.S. government agencies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	4,880
Gross Unrealized Gains	4
Gross Unrealized Losses	0
Available-for-sale investments	4,884	2,382
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	123,607	179,302
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 8,254	$ 9,623