Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 195,300	$ 445,133
Unrealized Gains	6	4
Unrealized (Losses)	(17)	(154)
Estimated Fair Value	195,289	444,983
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	6,254	22,074
Estimated Fair Value	6,254	22,074
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	133,112	242,089
Unrealized Gains	1	3
Unrealized (Losses)	(14)	(59)
Estimated Fair Value	133,099	242,033
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	55,934	180,970
Unrealized Gains	5	1
Unrealized (Losses)	(3)	(95)
Estimated Fair Value	55,936	180,876
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	64,998	171,310
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 130,291	257,713
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value		$ 15,960

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=65884222&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
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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+ References

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