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Stock-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 1,915,000 $ 942,000 $ 3,228,000 $ 1,794,000
Employee stock purchase plan | Employee Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future grant 1,616,342   1,616,342  
Shares reserved for grant, annual increase period     9 years  
Annual automatic increase in shares available, percentage of each class of common stock outstanding     1.00%  
Annual automatic increase in shares available, shares     1,500,000  
Purchase price of shares as a percentage of fair market value     90.00%  
Maximum number of shares participant may purchase, fair market value (not to exceed) $ 15,000   $ 15,000  
Maximum number of shares participant may purchase as a percentage of base compensation (not to exceed) 10.00%   10.00%  
Shares purchased by employees   18,705 13,584  
Stock-based compensation expense $ 100,000 $ 100,000 $ 100,000 $ 0
Employee stock purchase plan | Employee Stock | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Discount of the market value on the date of purchase (not to exceed) (percentage)     10.00%