Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	$ 36,709	$ 72,002
Warrant liability	2,576	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	36,709	72,002
Significant Other Observable In puts (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Warrant liability	$ 2,576	$ 12,962