Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investments	$ 1,789,213	$ 1,800,011
Cash	6,389	5,812
Total cash and investments in marketable securities	1,844,532	1,918,239
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	1,363,836	1,288,986
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	412,372	498,048
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	13,005	12,977
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	42,171	105,656
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in marketable securities	$ 6,759	$ 6,760