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Share-Based Compensation Expense - Summary of Equity Award Activity (Detail) (USD $)
9 Months Ended
Sep. 30, 2013
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Shares underlying option awards, Outstanding at beginning balance 3,585,743
Shares underlying option awards, Granted 3,765,504
Shares underlying option awards, Exercised   
Shares underlying option awards, Cancelled/forfeited/expired (203,144)
Shares underlying option awards, Outstanding at ending balance 7,148,103
Weighted average exercise price, beginning balance $ 2.31
Weighted average exercise price, Granted $ 0.51
Weighted average exercise price, option exercised   
Weighted average exercise price, Cancelled/forfeited/expired $ 0.74
Weighted average exercise price, ending balance $ 1.41