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Stock-Based Compensation - Restricted Stock Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Stock-based compensation    
Stock-based compensation expense | $ $ 3.1 $ 4.5
Non-employee director plan    
Stock-based compensation    
Total shares authorized 75,000  
Shares available for future grants 60,615  
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) 5,379  
Granted (in shares) 14,506 5,379
Vested (in shares) (8,379)  
Unvested as of end of period (in shares) 11,506 5,379
Weighted Average Grant Date Fair Value    
Unvested as of beginning of period (in dollars per share) | $ / shares $ 24.41  
Granted (in dollars per share) | $ / shares 29.74 $ 24.41
Vested (in dollars per share) | $ / shares 26.77  
Unvested as of end of period (in dollars per share) | $ / shares $ 29.74 $ 24.41
Stock-based compensation    
Total fair value of vested shares (in dollars) | $ $ 0.2  
Stock-based compensation expense | $ 0.3  
Unrecognized compensation expense | $ $ 0.2 $ 0.1
Period for recognition of unrecognized compensation expense 7 months 28 days  
Restricted Stock Units | Directors    
Number of Shares    
Granted (in shares) 9,006 5,379
Weighted Average Grant Date Fair Value    
Granted (in dollars per share) | $ / shares $ 30.19 $ 24.41
Stock-based compensation    
Vesting period   1 year
Restricted Stock Units | Employees    
Number of Shares    
Granted (in shares) 5,500  
Weighted Average Grant Date Fair Value    
Granted (in dollars per share) | $ / shares $ 28.97