Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 1,049,634	$ 1,089,419
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(644)	(381)
Cash, cash equivalents and short-term investments, Fair market value	1,048,991	1,089,040
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	507,802	532,999
Cash, cash equivalents and short-term investments, Fair market value	507,802	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	178,875	133,368
Cash, cash equivalents and short-term investments, Fair market value	178,875	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	270,422	294,915
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(304)	(200)
Cash, cash equivalents and short-term investments, Fair market value	270,119	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	92,535
Cash, cash equivalents and short-term investments, Gross unrealized losses	(340)
Cash, cash equivalents and short-term investments, Fair market value	92,195
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	557,233	599,698
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses		(9)
Cash, cash equivalents and short-term investments, Fair market value	557,234	599,691
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	492,401	479,705
Cash, cash equivalents and short-term investments, Gross unrealized losses	(644)	(336)
Cash, cash equivalents and short-term investments, Fair market value	$ 491,757	479,369
Corporate Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		118,121
Cash, cash equivalents and short-term investments, Gross unrealized losses		(145)
Cash, cash equivalents and short-term investments, Fair market value		117,976
Corporate Bonds - Non-current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term investments, Fair market value		9,980
Long-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term 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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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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
-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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