Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	$ 54,550	$ 72,002
Warrant liability	2,174	12,962
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	54,550	72,002
Significant Other Observable In puts (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Warrant liability	$ 2,174	$ 12,962