Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	$ 1,800,011	$ 347,326
Cash	5,812	4,460
Total cash and investments in marketable securities	1,918,239	353,220
Corporate Commercial Paper [Member] | Fair Value Hierarchy Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	498,048	128,096
Corporate Notes and Bonds [Member] | Fair Value Hierarchy Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	1,288,986	216,253
Obligations of U.S. government agencies [Member] | Fair Value Hierarchy Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	12,977	2,977
Money Market Funds [Member] | Fair Value Hierarchy Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	105,656	302
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 6,760	$ 1,132