Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Unrealized Gains		$ 2,100
Gross Unrealized Losses		(300)
Cash	$ 6,285	5,829
Total cash and investments in marketable securities	1,198,558	1,603,981
Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,002,443
Gross Unrealized Gains	1,081
Gross Unrealized Losses	(176)
Fair Value	1,003,348	1,507,655
Corporate Notes and Bonds | Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	686,543
Gross Unrealized Gains	1,035
Gross Unrealized Losses	(109)
Fair Value	687,469	1,132,182
Commercial Paper | Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	313,520
Gross Unrealized Gains	44
Gross Unrealized Losses	(67)
Fair Value	313,497	375,473
Obligations of U.S. government agencies | Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2,380
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Fair Value	2,382	0
Money Market Funds | Fair Value Hierarchy Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	179,302	83,546
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 9,623	$ 6,951