Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 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	$ 6,368	5,829
Total cash and investments in marketable securities	1,153,721	1,603,981
Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,087,369
Gross Unrealized Gains	2,473
Gross Unrealized Losses	(215)
Available-for-sale investments	1,089,627	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	831,575
Gross Unrealized Gains	2,341
Gross Unrealized Losses	(201)
Available-for-sale investments	833,715	1,132,182
Corporate commercial paper | Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	245,796
Gross Unrealized Gains	131
Gross Unrealized Losses	(14)
Available-for-sale investments	245,913	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	9,998
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Available-for-sale investments	9,999	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	48,931	83,546
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 8,795	$ 6,951