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Summary of Significant Accounting Policies - Securities Excluded from the Computation of Potentially Dilutive Securities (Detail) - shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share, amount 24,274,232 24,521,945
Employee Stock Option    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share, amount 5,693,250 5,693,250
Outstanding Restricted Stock Units    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share, amount 874,164 1,060,381
Outstanding Warrants    
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share, amount 17,706,818 17,768,314