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Stock-based Compensation - Narrative (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended
Jan. 31, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
May 31, 2015
Stock-based compensation              
Shares of common stock authorized for issuance outstanding (in shares)   4,140,629   4,140,629   3,585,856  
Employee Stock Purchase Plan, Purchase Price Percentage       85.00%      
Issuance for employee stock purchase plan, shares   41,316   74,142      
Proceeds from issuances of common stock from employee stock purchase plans   $ 373   $ 817 $ 1,117    
Employee Stock Purchase Plan, Compensation Expense   120 $ 156 293 398    
Share-based Compensation       12,562 $ 10,180    
Performance share units              
Stock-based compensation              
Vesting period 3 years            
Share-based Compensation   21   63      
Restricted Stock Awards And Restricted Stock Units              
Stock-based compensation              
Unrecognized compensation cost related to outstanding options   $ 31,051   $ 31,051      
Period over which unrecognized compensation cost is expected to be recognized as expense       2 years 7 months 6 days      
2014 Stock Incentive Plan              
Stock-based compensation              
Shares of common stock authorized for issuance (in shares)             2,700,000
Shares of common stock remaining available for future grant   1,251,722   1,251,722      
Increase in number of authorized shares on the first day of each fiscal year, as a percentage of outstanding common stock (as a percent)       4.00%      
Vesting period       4 years      
Contractual life       10 years      
2014 Stock Incentive Plan | Minimum              
Stock-based compensation              
Period following termination date vested options are exercisable       1 month      
2014 Stock Incentive Plan | Maximum              
Stock-based compensation              
Period following termination date vested options are exercisable       3 months