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Stock-Based Compensation - Options Valuation (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock-Based Compensation    
Total Stock-based compensation expense (in dollars) $ 1,945 $ 3,821
Stock options    
Stock-Based Compensation    
Total Stock-based compensation expense (in dollars) $ 1,135 $ 690
Employee stock options    
Valuation assumptions:    
Expected volatility (as a percent) 58.40% 61.00%
Expected life 6 years 29 days 6 years 11 days
Risk-free interest rate (as a percent) 2.32% 2.24%
Weighted average grant-date fair value (in dollars per share) $ 16.46 $ 7.89