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Common Stock Incentive Plan - Restricted Stock Units (Details) - Restricted Stock Units - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Common Stock Incentive Plan      
Vesting period (in years) 4 years 4 years 4 years
Remaining unamortized share-based compensation expense $ 399    
Number of shares      
Outstanding nonvested shares, beginning of year 15,511 14,968 24,869
Shares granted 9,966 5,482 8,383
Shares vested (6,807) (4,939) (10,118)
Shares forfeited 0 0 (8,166)
Outstanding nonvested shares, end of each period 18,670 15,511 14,968
Weighted average price      
Outstanding nonvested shares, beginning of year (in dollars per share) $ 59.82 $ 54.62 $ 55.97
Shares granted (in dollars per share) 78.79 69.58 49.96
Shares vested (in dollars per share) 58.63 54.90 55.01
Shares forfeited (in dollars per share) 0 0 53.45
Outstanding nonvested shares, end of each period (in dollars per share) $ 70.38 $ 59.82 $ 54.62