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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended
May 31, 2017
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options outstanding (in shares)   6,720,247   5,273,964
Weighted-average exercise price of stock options outstanding (in dollars per share)   $ 22.48   $ 22.23
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Total stock-based compensation expense   $ 3,799 $ 3,432  
2016 Employee Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation, number of shares authorized (in shares) 800,000      
Payroll deduction discount 10.00%      
Purchase price of common stock percent of the fair market value 85.00%      
Common stock purchased (in shares)   0    
2003 Stock Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options outstanding (in shares)   630,367    
Weighted-average exercise price of stock options outstanding (in dollars per share)   $ 2.19    
Stock Incentive Plan 2013        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation, number of shares authorized (in shares)   9,938,263    
Potential annual increase in shares reserved for future issuance (in shares)   1,960,168    
Potential annual increase in shares reserved for future issuance as percentage of outstanding share   4.00%    
Options outstanding (in shares)   6,089,880    
Weighted-average exercise price of stock options outstanding (in dollars per share)   $ 24.58    
Research and Development        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense   $ 1,814 1,700  
General and Administrative        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense   $ 1,985 $ 1,732