Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Additional Information (Detail)

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Stockholders' Equity - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2014
October 2009 Offering
Dec. 31, 2013
October 2009 Offering
Jun. 30, 2014
October 2009 Offering
Warrants
Jun. 30, 2014
October 2009 Offering
Warrants
Jun. 30, 2014
March 2011 Offering
Dec. 31, 2013
March 2011 Offering
Jun. 30, 2014
March 2011 Offering
Warrants
Jun. 30, 2014
March 2011 Offering
Warrants
Class Of Warrant Or Right [Line Items]                  
Warrants outstanding 8,300,000 4,287,940 4,287,940     3,993,783 3,993,783    
Warrants exercise price   2.05 2.05     2.46 2.46    
Change in fair value of common stock warrants       $ 3.4 $ 3.1     $ 3.2 $ 5.0