Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 435,669	$ 515,867
Unrealized Gains		2
Unrealized (Losses)	(800)	(411)
Estimated Fair Value	434,869	515,458
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	19,507	31,836
Estimated Fair Value	19,507	31,836
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	261,375	313,307
Unrealized Gains		2
Unrealized (Losses)	(425)	(145)
Estimated Fair Value	260,950	313,164
U.S. Treasury bills and government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	154,787	170,724
Unrealized (Losses)	(375)	(266)
Estimated Fair Value	154,412	170,458
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	152,198	162,793
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	240,157	281,589
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 42,514	$ 71,076

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
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain 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
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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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
-Topic 320
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-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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