Cash Equivalents and Short-term and Long-term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 296,133	$ 515,867
Unrealized Gains	2	2
Unrealized (Losses)	(286)	(411)
Estimated Fair Value	295,849	515,458
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,500	31,836
Estimated Fair Value	19,500	31,836
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	166,363	313,307
Unrealized Gains	1	2
Unrealized (Losses)	(205)	(145)
Estimated Fair Value	166,159	313,164
U.S. Treasury bills and government agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	110,270	170,724
Unrealized Gains	1
Unrealized (Losses)	(81)	(266)
Estimated Fair Value	110,190	170,458
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	117,836	162,793
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 178,013	281,589
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value		$ 71,076

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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