Financial Instruments that are Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Cash equivalents:
Total cash equivalents	$ 31,697	$ 17,000
Short-term investments:
Total short-term investments	54,649	32,874
Cash equivalents and short-term investments	86,346	49,874
Fair Value, Measurements, Recurring \| Money market mutual funds
Cash equivalents:
Total cash equivalents	31,697	17,000
Fair Value, Measurements, Recurring \| Corporate bonds
Short-term investments:
Total short-term investments	21,426	13,377
Fair Value, Measurements, Recurring \| U.S. agency securities
Short-term investments:
Total short-term investments	21,078	11,283
Fair Value, Measurements, Recurring \| Asset-backed securities
Short-term investments:
Total short-term investments	6,640	4,666
Fair Value, Measurements, Recurring \| U.S. Treasury securities
Short-term investments:
Total short-term investments	5,505	3,548
Quoted Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Total cash equivalents	31,697	17,000
Short-term investments:
Cash equivalents and short-term investments	31,697	17,000
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Money market mutual funds
Cash equivalents:
Total cash equivalents	31,697	17,000
Significant Other Observable Inputs (Level 2)
Short-term investments:
Total short-term investments	54,649	32,874
Cash equivalents and short-term investments	54,649	32,874
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate bonds
Short-term investments:
Total short-term investments	21,426	13,377
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| U.S. agency securities
Short-term investments:
Total short-term investments	21,078	11,283
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Asset-backed securities
Short-term investments:
Total short-term investments	6,639	4,666
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| U.S. Treasury securities
Short-term investments:
Total short-term investments	$ 5,506	$ 3,548

- Definition

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 Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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