Quarterly report pursuant to Section 13 or 15(d)

Common Stock - Additional Information (Detail)

v2.4.0.8
Common Stock - Additional Information (Detail) (USD $)
 1 Months Ended 9 Months Ended 87 Months Ended 9 Months Ended 9 Months Ended 0 Months Ended
May 31, 2013
Sep. 30, 2013
Plans
Sep. 30, 2012
Sep. 30, 2013
Plans
Jul. 31, 2013
Dec. 31, 2012
Jun. 19, 2013
2013 Plan [Member]
Jul. 31, 2013
2013 ATM [Member]
Apr. 30, 2013
2013 ATM [Member]
Sep. 30, 2013
Market Issuance Sales Agreement [Member]
Oct. 31, 2012
2012 ATM [Member]
Sep. 30, 2013
2012 and 2013 ATMs [Member]
Oct. 02, 2013
Subsequent Event [Member]
2013 ATM [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Common stock having an aggregate offering price     $ 75,000,000   $ 200,000,000       $ 45,000,000   $ 30,000,000    
Public offering of common stock, shares                       10,352,219 206,203
Net proceeds from public offering of common stock   90,629,000 28,750,000 119,682,000               88,000,000 1,400,000
Common stock having an aggregate offering price, additional amount               70,000,000          
Maximum percentage of fee paid under agreement                   3.00%      
Number of authorized shares, capital stock   115,000,000   115,000,000 65,000,000                
Number of authorized shares, common stock   100,000,000   100,000,000   50,000,000              
Number of equity compensation plans   3   3                  
Common Stock to be issued as equity grants             2,300,000            
Stock-based compensation expense related to all unvested stock option   9,100,000   9,100,000                  
Stock-based compensation weighted-average vesting period   1 year 6 months                      
Closing price of common stock $ 4.56 $ 7.01   $ 7.01                  
Stock issued during period shares employee stock purchase plans 21,505                        
Shares issued under employee stock purchase plan average price per share $ 3.88                        
Percentage of closing price for purchase price of common stock under employee stock purchase plan 85.00%                        
Issuance of common stock   80,705                      
Issued warrants to purchase common stock   158,429                      
Warrant exercise price   5.02   5.02                  
Shares of common stock   340   340                  
Issuance of Warrant related to Hercules Note   $ 1,098