Quarterly report pursuant to Section 13 or 15(d)

Common Stock - Summary of Stockholders Equity Activity (Detail)

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Common Stock - Summary of Stockholders Equity Activity (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 1 Months Ended 3 Months Ended 9 Months Ended 87 Months Ended
May 31, 2013
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Schedule Of Stockholders Equity [Line Items]            
Beginning Balance, Shares       24,400,754    
Exercise of Common Stock options, Shares       400,157    
Issuance of Common Stock related to the ESPP, Shares 21,505          
Ending Balance, Shares   35,255,680   35,255,680   35,255,680
Beginning Balance, Amount       $ 22,033    
Exercise of Common Stock options, Amount       680    
Sale of Common Stock under ATM programs, Amount       88,006    
Issuance of Common Stock related to the ESPP, Amount       83    
Stock-based compensation expense, Amount       4,072    
Net loss   (7,832) (5,935) (27,360) (18,945) (111,544)
Ending Balance, Amount   87,514   87,514   87,514
Common Shares [Member]
            
Schedule Of Stockholders Equity [Line Items]            
Beginning Balance, Shares       24,400,754    
Exercise of Common Stock options, Shares       400,157    
Exercise of Common Stock warrants, Shares       81,045    
Sale of Common Stock under ATM programs, Shares       10,352,219    
Issuance of Common Stock related to the ESPP, Shares       21,505    
Ending Balance, Shares   35,255,680   35,255,680   35,255,680
Beginning Balance, Amount       24    
Exercise of Common Stock options, Amount       1    
Sale of Common Stock under ATM programs, Amount       10    
Ending Balance, Amount   35   35   35
Additional Paid-in Capital [Member]
            
Schedule Of Stockholders Equity [Line Items]            
Beginning Balance, Amount       106,193    
Exercise of Common Stock options, Amount       679    
Sale of Common Stock under ATM programs, Amount       87,996    
Issuance of Common Stock related to the ESPP, Amount       83    
Stock-based compensation expense, Amount       4,072    
Ending Balance, Amount   199,023   199,023   199,023
Deficit Accumulated during Development Stage [Member]
            
Schedule Of Stockholders Equity [Line Items]            
Beginning Balance, Amount       (84,184)    
Net loss       (27,360)    
Ending Balance, Amount   $ (111,544)   $ (111,544)   $ (111,544)