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Stock-Based Compensation - Options Valuation (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Stock-Based Compensation        
Stock-based compensation expense (in dollars) $ 939 $ 223 $ 7,776 $ 481
Stock options        
Stock-Based Compensation        
Stock-based compensation expense (in dollars) $ 871 $ 120 $ 2,440 $ 378
Employee stock options        
Valuation assumptions:        
Expected volatility (as a percent)     61.00% 59.00%
Expected life     6 years 11 days 6 years 29 days
Risk-free interest rate (as a percent)     2.21% 1.49%
Weighted average grant-date fair value (in dollars per share)     $ 8.13 $ 7.29