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Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	$ 24,054	$ 17,482
Convertible preferred stock warrant liabilities	584	117
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	6,235	6,024
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	17,819	11,458
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible preferred stock warrant liabilities	584	117
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	5,733	2,973
Money market funds \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	5,733	2,973
U.S. treasury notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	502	3,051
U.S. treasury notes \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	502	3,051
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	4,248
Commercial paper \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	4,248
Corporate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	5,264	6,022
Corporate securities \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	5,264	6,022
U.S. government-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	8,307	5,436
U.S. government-backed securities \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Measurements	$ 8,307	$ 5,436

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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