Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax		$ 2,100
Debt securities, available-for-sale accumulated gross unrealized loss, before tax		(300)
Cash	$ 6,071	5,829
Total cash and investments in marketable securities	1,210,563	1,603,981
Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale, amortized cost	1,139,707
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax	4,570
Debt securities, available-for-sale accumulated gross unrealized loss, before tax	(170)
Available-for-sale investments	1,144,107	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]
Debt securities, available-for-sale, amortized cost	893,901
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax	4,095
Debt securities, available-for-sale accumulated gross unrealized loss, before tax	(143)
Available-for-sale investments	897,853	1,132,182
Corporate commercial paper | Fair Value Hierarchy Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale, amortized cost	235,811
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax	475
Debt securities, available-for-sale accumulated gross unrealized loss, before tax	(26)
Available-for-sale investments	236,260	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]
Debt securities, available-for-sale, amortized cost	9,995
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax	0
Debt securities, available-for-sale accumulated gross unrealized loss, before tax	(1)
Available-for-sale investments	9,994	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	52,135	83,546
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 8,250	$ 6,951