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Stock-Based Compensation - Restricted Stock Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Stock-based compensation      
Stock-based compensation expense | $ $ 7.8 $ 6.9 $ 3.1
Period for recognition of unrecognized compensation expense 1 year 3 months 18 days    
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) | shares 48,761 11,506 5,379
Granted (in shares) | shares 20,418 47,094 14,506
Vested (in shares) | shares (22,452) (9,839) (8,379)
Forfeited (in shares) | shares (1,944)    
Unvested as of end of period (in shares) | shares 44,783 48,761 11,506
Weighted Average Grant Date Fair Value      
Unvested as of beginning of period (in dollars per share) | $ / shares $ 28.60 $ 29.74 $ 24.41
Granted (in dollars per share) | $ / shares 26.50 28.64 29.74
Vested (in dollars per share) | $ / shares 28.37 32.31 26.77
Forfeited (in dollars per share) | $ / shares 28.29    
Unvested as of end of period (in dollars per share) | $ / shares $ 27.82 $ 28.60 $ 29.74
Stock-based compensation      
Total fair value of vested shares (in dollars) | $ $ 0.6 $ 0.3 $ 0.2
Unrecognized compensation expense | $ $ 0.9 $ 1.1 $ 0.2
Period for recognition of unrecognized compensation expense 1 year    
Restricted Stock Units | Directors      
Number of Shares      
Granted (in shares) | shares 15,918 11,799 9,006
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares $ 26.38 $ 27.89 $ 30.19
Restricted Stock Units | Employees      
Number of Shares      
Granted (in shares) | shares 4,500 35,295 5,500
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares $ 26.92 $ 28.89 $ 28.97