Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 732,179	$ 370,827
Cash and cash equivalents, Gross unrealized gains	11	3
Cash and cash equivalents, Gross unrealized losses		(1)
Cash and cash equivalents	732,190	370,829	$ 557,234
Cash, cash equivalents and investments, Amortized cost	1,344,189	1,174,011
Cash, cash equivalents and investments, Gross unrealized gains	31	12
Cash, cash equivalents and investments, Gross unrealized losses	(12)	(111)
Cash, cash equivalents and investments, Fair market value	1,344,208	1,173,912
Available for sale debt securities current, Amortized cost	612,010	803,184
Available for sale debt securities current, Gross unrealized gains	20	9
Available for sale debt securities current, Gross unrealized losses	(12)	(110)
Available for sale debt securities current, Fair market value	612,018	803,083
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	437,822	244,302
Cash and cash equivalents	437,822	244,302
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	65,779	125,907
Available for sale debt securities, Fair market value	65,779	125,907
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	807,001	760,258
Available for sale debt securities, Gross unrealized gains	31	12
Available for sale debt securities, Gross unrealized losses	(7)	(35)
Available for sale debt securities, Fair market value	807,025	760,235
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	33,587	43,544
Available for sale debt securities, Gross unrealized losses	(5)	(76)
Available for sale debt securities, Fair market value	$ 33,582	$ 43,468

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

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