Financial Instruments that are Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Short-term investments:
Total short-term investments	$ 103,604	$ 32,874
Fair Value, Measurements, Recurring
Cash equivalents:
Total cash equivalents	142,968	17,000
Short-term investments:
Total short-term investments	103,604	32,874
Cash equivalents and short-term investments	246,572	49,874
Fair Value, Measurements, Recurring | Money market mutual funds
Cash equivalents:
Total cash equivalents	142,968	17,000
Fair Value, Measurements, Recurring | Corporate bonds
Short-term investments:
Total short-term investments	39,555	13,377
Fair Value, Measurements, Recurring | U.S. agency securities
Short-term investments:
Total short-term investments	32,274	11,283
Fair Value, Measurements, Recurring | Asset-backed securities
Short-term investments:
Total short-term investments	13,957	4,666
Fair Value, Measurements, Recurring | U.S. Treasury securities
Short-term investments:
Total short-term investments	17,818	3,548
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Cash equivalents:
Total cash equivalents	142,968	17,000
Short-term investments:
Cash equivalents and short-term investments	142,968	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	142,968	17,000
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Short-term investments:
Total short-term investments	103,604	32,874
Cash equivalents and short-term investments	103,604	32,874
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate bonds
Short-term investments:
Total short-term investments	39,555	13,377
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. agency securities
Short-term investments:
Total short-term investments	32,274	11,283
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Asset-backed securities
Short-term investments:
Total short-term investments	13,957	4,666
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. Treasury securities
Short-term investments:
Total short-term investments	$ 17,818	$ 3,548