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Stock-Based Compensation - Employee expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-based compensation      
Total employee, director and non-employee stock-based compensation expense $ 13,651 $ 7,848 $ 4,869
General and administrative      
Stock-based compensation      
Total employee, director and non-employee stock-based compensation expense 5,617 3,592 2,218
Research and development      
Stock-based compensation      
Total employee, director and non-employee stock-based compensation expense $ 8,034 $ 4,256 $ 2,650