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Stock-Based Compensation - Restricted Stock Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Stock-based compensation      
Stock-based compensation expense | $ $ 10.0 $ 7.8 $ 6.9
Period for recognition of unrecognized compensation expense 1 year 1 month 6 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 44,783 48,761 11,506
Granted (in shares) | shares 28,783 20,418 47,094
Vested (in shares) | shares (30,016) (22,452) (9,839)
Forfeited (in shares) | shares   (1,944)  
Unvested as of end of period (in shares) | shares 43,550 44,783 48,761
Weighted Average Grant Date Fair Value      
Unvested as of beginning of period (in dollars per share) | $ / shares $ 27.82 $ 28.60 $ 29.74
Granted (in dollars per share) | $ / shares 15.86 26.50 28.64
Vested (in dollars per share) | $ / shares 27.47 28.37 32.31
Forfeited (in dollars per share) | $ / shares   28.29  
Unvested as of end of period (in dollars per share) | $ / shares $ 21.93 $ 27.82 $ 28.60
Stock-based compensation      
Total fair value of vested shares (in dollars) | $ $ 0.8 $ 0.6 $ 0.3
Unrecognized compensation expense | $ $ 0.4 $ 0.9 $ 1.1
Period for recognition of unrecognized compensation expense 6 months    
Restricted Stock Units | Directors      
Number of Shares      
Granted (in shares) | shares 28,783 15,918 11,799
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares $ 15.86 $ 26.38 $ 27.89
Restricted Stock Units | Employees      
Number of Shares      
Granted (in shares) | shares 0 4,500 35,295
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) | $ / shares   $ 26.92 $ 28.89