Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 207,227	$ 198,226
Gross unrealized gain	149	0
Gross unrealized loss	(1)	(40)
Available-for-sale securities, fair value	207,375	198,186
Cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, fair value	43,082	145,747
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, fair value	164,293	50,688
Long-term investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, fair value	0	1,751
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	43,082	145,747
Gross unrealized gain	0	0
Gross unrealized loss	0	0
Available-for-sale securities, fair value	43,082	145,747
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	150,207	0
Gross unrealized gain	146	0
Gross unrealized loss	0	0
Available-for-sale securities, fair value	150,353	0
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13,938	52,479
Gross unrealized gain	3	0
Gross unrealized loss	(1)	(40)
Available-for-sale securities, fair value	$ 13,940	$ 52,439

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
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-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.

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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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