Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 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	$ 14,056	6,285
Total cash and investments in marketable securities	1,057,462	1,198,558
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	896,784
Gross Unrealized Gains	205
Gross Unrealized Losses	(127)
Fair Value	896,862	1,003,348
Corporate notes and bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	403,490
Gross Unrealized Gains	152
Gross Unrealized Losses	(91)
Fair Value	403,551	687,469
Corporate commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	487,414
Gross Unrealized Gains	51
Gross Unrealized Losses	(35)
Fair Value	487,430	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	5,880
Gross Unrealized Gains	2
Gross Unrealized Losses	(1)
Fair Value	5,881	2,382
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	138,289	179,302
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 8,255	$ 9,623