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Stock-Based Compensation (Details) - USD ($)
1 Months Ended 3 Months Ended
Dec. 31, 2018
Jun. 30, 2017
Mar. 31, 2019
Mar. 31, 2018
Jan. 31, 2019
Jun. 30, 2015
Stock Based Compensation            
Stock-based compensation expense     $ 4,051,000 $ 3,154,000    
Chief Executive Officer            
Stock Based Compensation            
Stock option shares approved   425,000        
2015 Plan            
Stock Based Compensation            
Shares reserved for issuance           3,451,495
Additional shares reserved for issuance           9,890,310
Performance-based awards            
Stock Based Compensation            
Stock option shares approved 600,000          
Stock-based compensation expense $ 0   0      
Performance-based awards | Chief Executive Officer            
Stock Based Compensation            
Stock option shares approved   200,000     200,000  
Requisite service period   3 years        
Stock-based compensation expense     $ 200,000 $ 200,000    
Performance-based awards | First Vesting Milestone | Chief Executive Officer            
Stock Based Compensation            
Number of options vested   50,000        
Vesting period   1 year        
Performance-based awards | Second Vesting Milestone | Chief Executive Officer            
Stock Based Compensation            
Number of options vested   75,000        
Vesting period   1 year        
Performance-based awards | Third Vesting Milestone | Chief Executive Officer            
Stock Based Compensation            
Number of options vested   75,000        
Vesting period   1 year        
Employee and non-employee stock options            
Stock Based Compensation            
Shares available for issuance 5,431,000   6,774,000      
Restricted stock units            
Stock Based Compensation            
Shares granted     1,202,000      
Restricted stock units | Chief Executive Officer            
Stock Based Compensation            
Stock option shares approved         100,000  
2015 Employee Stock Purchase Plan            
Stock Based Compensation            
Shares reserved for issuance     893,548      
Shares reserved for issuance as a proportion common stock outstanding (as a percent)     1.00%      
Price in relation to fair market value of common stock on the date of grant, lower range limit (as a percent)     85.00%      
Maximum number of shares a participant may receive during the period (in shares)     5,000      
Maximum amount of award or purchase during a calendar year     $ 25,000      
Maximum employee contribution of employee's cash compensation (as a percent)     15.00%      
2015 Employee Stock Purchase Plan | Minimum            
Stock Based Compensation            
Shares reserved for issuance     880,000