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Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense Included in Statements of Operations and Comprehensive Loss (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Total stock-based compensation expense $ 4,415 $ 3,247
Cost of Revenue    
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Total stock-based compensation expense 88 117
Research and development    
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Total stock-based compensation expense 873 576
Selling, general and administrative    
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]    
Total stock-based compensation expense $ 3,454 $ 2,554