Summary of Company Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 370,827	$ 599,698
Cash and cash equivalents, Gross unrealized gains	3	2
Cash and cash equivalents, Gross unrealized losses	(1)	(9)
Cash and cash equivalents	370,829	599,691	$ 122,420
Cash, cash equivalents and investments, Amortized cost	1,174,011	1,089,419
Cash, cash equivalents and investments, Gross unrealized gains	12	2
Cash, cash equivalents and investments, Gross unrealized losses	(111)	(381)
Cash, cash equivalents and investments, Fair market value	1,173,912	1,089,040
Available for sale debt securities current, Amortized cost	803,184	479,705
Available for sale debt securities current, Gross unrealized gains	9
Available for sale debt securities current, Gross unrealized losses	(110)	(336)
Available for sale debt securities current, Fair market value	803,083	479,369
Available for sale debt securities noncurrent, Amortized cost		10,016
Available for sale debt securities noncurrent, Gross unrealized losses		(36)
Available for sale debt securities noncurrent, Fair market value		9,980
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	244,302	532,999
Cash and cash equivalents	244,302	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	125,907	133,368
Available for sale debt securities, Fair market value	125,907	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	760,258	294,915
Available for sale debt securities, Gross unrealized gains	12	2
Available for sale debt securities, Gross unrealized losses	(35)	(200)
Available for sale debt securities, Fair market value	760,235	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	43,544
Available for sale debt securities, Gross unrealized losses	(76)
Available for sale debt securities, Fair market value	$ 43,468
Available for sale debt securities current, Amortized cost		118,121
Available for sale debt securities current, Gross unrealized losses		(145)
Available for sale debt securities current, Fair market value		117,976
Available for sale debt securities noncurrent, Amortized cost		10,016
Available for sale debt securities noncurrent, Gross unrealized losses		(36)
Available for sale debt securities noncurrent, Fair market value		$ 9,980

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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