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Share-Based Compensation - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jan. 01, 2019
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Stock options          
Stock Option and Compensation Plans          
Unrecognized share-based compensation costs for option awards, net of estimated forfeitures   $ 6,400   $ 6,400  
Expected weighted average period to recognize share-based compensation costs       1 year 9 months 18 days  
Restricted stock units          
Stock Option and Compensation Plans          
Expected weighted average period to recognize share-based compensation costs       1 year 6 months  
Unrecognized compensation costs, net of estimated forfeitures   $ 2,100   $ 2,100  
ESPP          
Stock Option and Compensation Plans          
Common stock issued (in shares)   0 0 31,522 12,229
Proceeds from stock plans   $ 41 $ 27    
2013 Plan          
Stock Option and Compensation Plans          
Increase in number of shares available under the Plan (in shares) 1,656,000     8,554,000  
Annual increase in shares reserved for issuance under the Plan (as a percent) 4.00%        
Number of shares of common stock available for issuance under the Plan (in shares)   2,704,000   2,704,000  
2013 Plan | ESPP          
Stock Option and Compensation Plans          
Number of shares available for future purchase (in shares)   920,707   920,707