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Share-based compensation - Additional Information (Detail) - USD ($)
1 Months Ended 3 Months Ended
Jun. 07, 2018
Sep. 10, 2004
Sep. 17, 2014
Aug. 27, 2008
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]              
Share based compensation expense         $ 377,000 $ 27,000  
Issuance of stock option to purchase stock         7,000,000    
Common stock reserved for issuance         2,884,509    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value         $ 3,600,000   $ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value, Nonvested         1,400,000   $ 0
Share based Compensation Arrangement By Share based Payment Award Options Exercisable And Vested Intrinsic Value1         $ 0 $ 13,950  
Options granted         258,500 20,000  
2004 Stock Incentive Plan              
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]              
Common stock reserved for issuance   2,500,000          
Plan expiration date   Sep. 10, 2004          
2008 Stock Incentive Plan              
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]              
Common stock reserved for issuance       3,000,000      
Plan expiration date       Aug. 27, 2008      
2014 Stock Incentive Plan              
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]              
Common stock reserved for issuance     3,000,000        
Share based compensation arrangement by share based payment award percentage of fair market Person holding more then 10% voting stock     110.00%        
2018 Stock Incentive Plan              
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]              
Common stock reserved for issuance 4,000,000            
Plan expiration date Jun. 07, 2028            
Share based compensation arrangement by share based payment award percentage of fair market Person holding more then 10% voting stock 110.00%