Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 808,562	$ 370,827
Cash and cash equivalents, Gross unrealized gains	34	3
Cash and cash equivalents, Gross unrealized losses	(5)	(1)
Cash and cash equivalents	808,591	370,829	$ 410,375
Cash, cash equivalents and short-term investments, Amortized cost	1,102,987	1,174,011
Cash, cash equivalents and short-term investments, Gross unrealized gains	97	12
Cash, cash equivalents and short-term investments, Gross unrealized losses	(15)	(111)
Cash, cash equivalents and short-term investments, Fair value	1,103,069	1,173,912
Available for sale debt securities current, Amortized cost	294,425	803,184
Available for sale debt securities current, Gross unrealized gains	63	9
Available for sale debt securities current, Gross unrealized losses	(10)	(110)
Available for sale debt securities current, Fair value	294,478	803,083
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	318,475	244,302
Cash and cash equivalents	318,475	244,302
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	87,370	125,907
Available for sale debt securities, Fair value	87,370	125,907
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	677,166	760,258
Available for sale debt securities, Gross unrealized gains	91	12
Available for sale debt securities, Gross unrealized losses	(15)	(35)
Available for sale debt securities, Fair value	677,242	760,235
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	19,976	43,544
Available for sale debt securities, Gross unrealized gains	6
Available for sale debt securities, Gross unrealized losses		(76)
Available for sale debt securities, Fair value	$ 19,982	$ 43,468

Available for sale debt securities current accumulated gross unrealized gain before tax.

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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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