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Stock-Based Compensation - Restricted Stock Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Stock-based compensation      
Stock-based compensation expense | $ $ 6.9 $ 3.1 $ 4.5
Non-employee director plan      
Stock-based compensation      
Total shares authorized 75,000    
Shares available for future grants 48,816    
Restricted Stock Units      
Stock-based compensation      
Ratio of common stock for each vested award 1    
Number of Shares      
Unvested as of beginning of period (in shares) 11,506 5,379  
Granted (in shares) 47,094 14,506 5,379
Vested (in shares) (9,839) (8,379)  
Unvested as of end of period (in shares) 48,761 11,506 5,379
Weighted Average Grant Date Fair Value      
Unvested as of beginning of period (in dollars per share) | $ / shares $ 29.74 $ 24.41  
Granted (in dollars per share) | $ / shares 28.64 29.74 $ 24.41
Vested (in dollars per share) | $ / shares 32.31 26.77  
Unvested as of end of period (in dollars per share) | $ / shares $ 28.60 $ 29.74 $ 24.41
Stock-based compensation      
Total fair value of vested shares (in dollars) | $ $ 0.3 $ 0.2  
Stock-based compensation expense | $ 0.4 0.3  
Unrecognized compensation expense | $ $ 1.1 $ 0.2  
Period for recognition of unrecognized compensation expense 1 year 4 months 24 days    
Restricted Stock Units | Directors      
Number of Shares      
Granted (in shares) 11,799 9,006 5,379
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares $ 27.89 $ 30.19 $ 24.41
Stock-based compensation      
Vesting period     1 year
Restricted Stock Units | Employees      
Number of Shares      
Granted (in shares) 35,295 5,500  
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares $ 28.89 $ 28.97