Summary of Company Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	$ 1,089,419	$ 317,965
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(381)	(120)
Cash, cash equivalents and investments, Fair market value	1,089,040	317,845
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	532,999	122,420
Cash, cash equivalents and investments, Fair market value	532,999	122,420
Commercial Paper - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	133,368	69,355
Cash, cash equivalents and investments, Gross unrealized losses		(51)
Cash, cash equivalents and investments, Fair market value	133,368	69,304
Government and Government Agency Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	294,915	105,340
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(200)	(53)
Cash, cash equivalents and investments, Fair market value	294,717	105,287
Corporate Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	118,121	20,850
Cash, cash equivalents and investments, Gross unrealized losses	(145)	(16)
Cash, cash equivalents and investments, Fair market value	117,976	20,834
Corporate Bonds - Non-current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	10,016
Cash, cash equivalents and investments, Gross unrealized losses	(36)
Cash, cash equivalents and investments, Fair market value	9,980
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	599,698	122,420
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(9)
Cash, cash equivalents and investments, Fair market value	599,691	122,420
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	479,705	195,545
Cash, cash equivalents and investments, Gross unrealized losses	(336)	(120)
Cash, cash equivalents and investments, Fair market value	479,369	$ 195,425
Long-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	10,016
Cash, cash equivalents and investments, Gross unrealized losses	(36)
Cash, cash equivalents and investments, Fair market value	$ 9,980

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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