Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 724,808	$ 370,827
Cash and cash equivalents, Gross unrealized gains	24	3
Cash and cash equivalents, Gross unrealized losses	(3)	(1)
Cash and cash equivalents	724,829	370,829	$ 209,702
Cash, cash equivalents and short-term investments, Amortized cost	1,048,817	1,174,011
Cash, cash equivalents and short-term investments, Gross unrealized gains	78	12
Cash, cash equivalents and short-term investments, Gross unrealized losses	(3)	(111)
Cash, cash equivalents and short-term investments, Fair value	1,048,892	1,173,912
Available for sale debt securities current, Amortized cost	324,009	803,184
Available for sale debt securities current, Gross unrealized gains	54	9
Available for sale debt securities current, Gross unrealized losses		(110)
Available for sale debt securities current, Fair value	324,063	803,083
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	303,090	244,302
Cash and cash equivalents	303,090	244,302
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	9,984	125,907
Available for sale debt securities, Fair value	9,984	125,907
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	725,749	760,258
Available for sale debt securities, Gross unrealized gains	74	12
Available for sale debt securities, Gross unrealized losses	(3)	(35)
Available for sale debt securities, Fair value	725,820	760,235
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	9,994	43,544
Available for sale debt securities, Gross unrealized gains	4
Available for sale debt securities, Gross unrealized losses		(76)
Available for sale debt securities, Fair value	$ 9,998	$ 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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