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Stock-Based Compensation - Restricted Stock Unit Activity (Details) - Restricted stock units - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of RSUs      
Beginning balance (in shares) 61,482    
Granted (in shares) 534,146 61,482 0
Vested (in shares) 0    
Forfeited (in shares) (37,360)    
Ending balance (in shares) 558,268 61,482  
Weighted Average Grant Date Fair Value      
Beginning balance (USD per share) $ 30.00    
Granted (USD per share) 35.03    
Vested (USD per share) 0.00    
Forfeited (USD per share) 31.56    
Ending balance (USD per share) $ 34.71 $ 30.00  
Aggregate Intrinsic Value (in thousands)      
Beginning balance $ 1,711    
Granted    
Vested 0    
Forfeited    
Ending balance $ 21,075 $ 1,711  
Vested and expected to vest (in shares) 558,268    
Vested and expected to vest (USD per share) $ 34.71    
Vested and expected to vest, aggregate intrinsic value $ 21,075