Financial Instruments that are Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 98,922	$ 170,132	$ 174,674	$ 76,568
Short-term investments:
Total short-term investments	125,001	105,001
Convertible Notes Payable
Short-term investments:
Fair value of convertible notes	113,438	141,200
Fair Value, Measurements, Recurring
Cash equivalents:
Total cash equivalents	635	164
Short-term investments:
Total short-term investments	125,001	105,001
Cash equivalents and short-term investments	125,636	105,165
Fair Value, Measurements, Recurring | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	635
Cash equivalents:
Total cash equivalents		164
Fair Value, Measurements, Recurring | Corporate bonds
Short-term investments:
Total short-term investments		40,583
Fair Value, Measurements, Recurring | U.S. agency securities
Short-term investments:
Total short-term investments		31,747
Fair Value, Measurements, Recurring | Asset-backed securities
Short-term investments:
Total short-term investments		15,882
Fair Value, Measurements, Recurring | U.S. Treasury securities
Short-term investments:
Total short-term investments		16,789
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Cash equivalents:
Total cash equivalents	635	164
Short-term investments:
Cash equivalents and short-term investments	635	164
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Money market mutual funds
Cash equivalents:
Total cash equivalents	635	164
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Short-term investments:
Total short-term investments	125,001	105,001
Cash equivalents and short-term investments	125,001	105,001
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate bonds
Short-term investments:
Total short-term investments	47,944	40,583
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. agency securities
Short-term investments:
Total short-term investments	39,277	31,747
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Asset-backed securities
Short-term investments:
Total short-term investments	22,008	15,882
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. Treasury securities
Short-term investments:
Total short-term investments	$ 15,772	$ 16,789