Cash Equivalents and Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 445,133	$ 371,370
Unrealized Gains	4	12
Unrealized (Losses)	(154)	(251)
Estimated Fair Value	444,983	371,131
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	22,074	24,915
Estimated Fair Value	22,074	24,915
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	242,089	226,209
Unrealized Gains	3	8
Unrealized (Losses)	(59)	(170)
Estimated Fair Value	242,033	226,047
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	180,970	120,246
Unrealized Gains	1	4
Unrealized (Losses)	(95)	(81)
Estimated Fair Value	180,876	120,169
Cash equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	171,310	36,341
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	257,713	251,759
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 15,960	$ 83,030

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=51813488&loc=d3e22054-111558

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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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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+ References

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