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Stock-Based Compensation - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Number ofStock Options    
Outstanding, beginning of period (in shares) 327,270  
Exercised (in shares) (8,500)  
Canceled/Forfeited (in shares) 500  
Outstanding, end of period (in shares) 318,270 327,270
Exercisable (in shares) 296,738  
Weighted AverageExercise Price    
Outstanding, beginning of period (in USD per share) $ 20.21  
Exercised (in USD per share) 18.51  
Canceled/Forfeited (in USD per share) 28.15  
Outstanding, end of period (in USD per share) 20.24 $ 20.21
Exercisable (in USD per share) $ 19.67  
Weighted 
Average 
Remaining 
Contractual Life (years)    
Outstanding 3 years 9 months 25 days 4 years 3 months 10 days
Exercisable 3 years 5 months 19 days  
Aggregate 
Intrinsic 
Value
(in thousands)    
Outstanding, beginning of period $ 2,255  
Exercised 86  
Outstanding, end of period 1,579 $ 2,255
Exercisable $ 1,579