Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 792,852	$ 1,089,419
Cash, cash equivalents and short-term investments, Gross unrealized gains	213	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(205)	(381)
Cash, cash equivalents and short-term investments, Fair market value	792,860	1,089,040
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	109,776	532,999
Cash, cash equivalents and short-term investments, Fair market value	109,776	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	89,795	133,368
Cash, cash equivalents and short-term investments, Fair market value	89,795	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	509,944	294,915
Cash, cash equivalents and short-term investments, Gross unrealized gains	213	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(129)	(200)
Cash, cash equivalents and short-term investments, Fair market value	510,028	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	83,337
Cash, cash equivalents and short-term investments, Gross unrealized losses	(76)
Cash, cash equivalents and short-term investments, Fair market value	83,261
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	209,576	599,698
Cash, cash equivalents and short-term investments, Gross unrealized gains	126	2
Cash, cash equivalents and short-term investments, Gross unrealized losses		(9)
Cash, cash equivalents and short-term investments, Fair market value	209,702	599,691
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	583,276	479,705
Cash, cash equivalents and short-term investments, Gross unrealized gains	87
Cash, cash equivalents and short-term investments, Gross unrealized losses	(205)	(336)
Cash, cash equivalents and short-term investments, Fair market value	$ 583,158	479,369
Corporate Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		118,121
Cash, cash equivalents and short-term investments, Gross unrealized losses		(145)
Cash, cash equivalents and short-term investments, Fair market value		117,976
Corporate Bonds - Non-current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term investments, Fair market value		9,980
Long-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term investments, Fair market value		$ 9,980