Quarterly report pursuant to Section 13 or 15(d)

Stock Based Compensation - Summary of Stock-based Compensation Reflected within Statements of Operations (Detail)

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Stock Based Compensation - Summary of Stock-based Compensation Reflected within Statements of Operations (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Share-based Compensation, Total $ 3,852,656 $ 4,568,716 $ 12,804,744 $ 12,472,870
Research and Development Expense [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Allocated share-based compensation expense 2,051,986 2,558,472 7,594,209 6,643,094
Selling and Marketing Expense [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Allocated share-based compensation expense 352,941 281,518 1,022,832 782,366
General and Administrative Expense [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]        
Allocated share-based compensation expense $ 1,447,729 $ 1,728,726 $ 4,187,703 $ 5,047,410