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Summary of Significant Accounting Policies - Additional Information (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended
Jul. 31, 2015
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
Segment
Dec. 31, 2017
USD ($)
Jan. 01, 2018
USD ($)
Aug. 28, 2017
Summary Of Significant Accounting Policies [Line Items]                          
Revenue, practical expedient, incremental cost of obtaining contract [true/false]                   true      
Revenue, practical expedient, remaining performance obligation, description                   The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer      
Cash equivalents maturity                   three months or less      
Cash equivalents   $ 300,000       $ 32,700,000       $ 300,000 $ 32,700,000    
Cash held in a separate restricted bank account   1,097,000       1,097,000       1,097,000 1,097,000    
Transfers of assets and liabilities between Level 1, Level 2, or Level 3                   0 0    
Impairment charge                   $ 0      
Lease expiration date 2021-04                 2017-07      
Incentive unit liquidation value   0               $ 0      
Description of tax benefit likely to be realized upon settlement                   greater than 50%      
Exchange ratio of common stock                         0.5532
Number of operating segment | Segment                   1      
Revenue   111,000 $ 1,801,000 $ 254,000 $ 354,000 111,000 $ 111,000 $ 2,111,000 $ 111,000 $ 2,520,000 2,444,000    
ASC 606                          
Summary Of Significant Accounting Policies [Line Items]                          
Cumulative-effect decrease to opening accumulated deficit                       $ 300,000  
Deferred revenue   300,000               300,000      
Revenue   2,500,000                      
ASC 606 | Impact as Result of Adoption of ASC 606                          
Summary Of Significant Accounting Policies [Line Items]                          
Deferred revenue   (300,000)               (300,000)      
ASC 606 | Balances without Adoption of ASC 606                          
Summary Of Significant Accounting Policies [Line Items]                          
Deferred revenue   700,000               700,000      
Revenue   2,400,000                      
Bank Account                          
Summary Of Significant Accounting Policies [Line Items]                          
Cash held in a separate restricted bank account   50,000       $ 50,000       50,000 $ 50,000    
Letter of Credit                          
Summary Of Significant Accounting Policies [Line Items]                          
Cash   $ 1,000,000               $ 1,000,000