Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	$ 109,455	$ 103,736
Warrant liability	25,472	19,425
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash equivalents	109,455	103,736
Significant Other Observable In puts (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Warrant liability	$ 25,472	$ 19,425