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Notes Payable - Additional Information (Detail) - Loan and Security Agreement [Member] - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Mar. 24, 2017
Oct. 28, 2016
Mar. 31, 2017
Oct. 31, 2016
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
LIBOR [Member]              
Debt Instrument [Line Items]              
Debt instrument, interest rate floor       0.50%      
Horizon Technology Finance Corporation and Silicon Valley Bank [Member]              
Debt Instrument [Line Items]              
Loan borrowed amount       $ 20.0      
Debt instrument, interest rate floor       10.50%      
Variable rate basis         one-month LIBOR    
Interest rate description         Each loan tranche bears interest at a floating rate equal to 10.5% plus the amount by which the one-month London Interbank Offer Rate (“LIBOR”) exceeds 0.5%.    
Minimum cash balance required in deposit account       $ 2.5      
Additional default interest rate       5.00%      
Warrant exercise price condition         Exercise price equal to the lower of (a) the volume weighted average price per share of the Company’s Class A Common Stock, as reported on the principal stock exchange on which the Company’s Class A Common Stock is listed, for 10 trading days prior to the issuance of the applicable Warrants or (b) the closing price of a share of the Company’s Class A Common Stock on the trading day prior to the issuance of the applicable Warrants.    
Warrants expiration period   7 years          
Costs incurred in connection with loan agreement             $ 0.7
Allocated fair value of warrants issued         $ 0.9 $ 0.9  
Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Prepayments for First 18 Months [Member]              
Debt Instrument [Line Items]              
Prepayment penalty fee       4.00%      
Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Prepayments Thereafter [Member]              
Debt Instrument [Line Items]              
Prepayment penalty fee       2.00%      
Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche One [Member]              
Debt Instrument [Line Items]              
Loan borrowed amount       $ 12.5      
Debt instrument, payment terms         The Company borrowed the first tranche of $12.5 million upon close of the Loan Agreement in October 2016. The first tranche requires only monthly interest payments until May 1, 2018 followed by equal monthly payments of principal plus accrued interest through the scheduled maturity date on May 1, 2020.    
Debt instrument, final payment       $ 0.8      
Debt instrument, maturity date       May 01, 2020      
Debt instrument, frequency of periodic Payment         monthly    
Warrants to purchase shares of common stock   152,580          
Exercise price of warrants   $ 6.39          
Warrant shares percentage issued of loan amount   6.00%          
Final payments, accrued additional interest expense         $ 0.8    
Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche Two [Member]              
Debt Instrument [Line Items]              
Loan borrowed amount     $ 7.5        
Debt instrument, payment terms         The Company borrowed the second tranche of $7.5 million in March 2017. The second tranche requires only monthly interest payments until October 1, 2018 followed by equal monthly payments of principal plus accrued interest through the scheduled maturity date on October 1, 2020.    
Debt instrument, final payment     $ 0.5        
Debt instrument, maturity date     Oct. 01, 2020        
Debt instrument, frequency of periodic Payment         monthly    
Warrants to purchase shares of common stock 38,006            
Exercise price of warrants $ 5.92            
Warrant shares percentage issued of loan amount 3.00% 3.00%          
Final payments, accrued additional interest expense         $ 0.5    
Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche Three [Member]              
Debt Instrument [Line Items]              
Loan amount available for borrowing         $ 5.0    
Tranches expiration date         Jun. 30, 2017