Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 949,505	$ 1,089,419
Cash, cash equivalents and short-term investments, Gross unrealized gains	5	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(366)	(381)
Cash, cash equivalents and short-term investments, Fair market value	949,144	1,089,040
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	395,416	532,999
Cash, cash equivalents and short-term investments, Fair market value	395,416	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	189,622	133,368
Cash, cash equivalents and short-term investments, Fair market value	189,622	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	278,680	294,915
Cash, cash equivalents and short-term investments, Gross unrealized gains	5	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(173)	(200)
Cash, cash equivalents and short-term investments, Fair market value	278,512	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	85,787
Cash, cash equivalents and short-term investments, Gross unrealized losses	(193)
Cash, cash equivalents and short-term investments, Fair market value	85,594
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	410,375	599,698
Cash, cash equivalents and short-term investments, Gross unrealized gains		2
Cash, cash equivalents and short-term investments, Gross unrealized losses		(9)
Cash, cash equivalents and short-term investments, Fair market value	410,375	599,691
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	539,130	479,705
Cash, cash equivalents and short-term investments, Gross unrealized gains	5
Cash, cash equivalents and short-term investments, Gross unrealized losses	(366)	(336)
Cash, cash equivalents and short-term investments, Fair market value	$ 538,769	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