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Equity Incentive Plan - Performance share activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2020
Performance Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares Outstanding, at beginning of the period   31    
Shares Outstanding, granted   30 31  
Shares Outstanding, at end of the period     31  
Weighted Average Grant Date Fair Value, at beginning of the period (in dollars per share)   $ 55.29    
Weighted Average Grant Date Fair Value, granted (in dollars per share)   66.96 $ 55.29  
Weighted Average Grant Date Fair Value, at end of the period (in dollars per share)   $ 55.29 $ 55.29  
Performance Units And Shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares Outstanding, at beginning of the period 61      
Shares Outstanding, granted 26      
Shares Outstanding, at end of the period 87 61    
Weighted Average Grant Date Fair Value, at beginning of the period (in dollars per share) $ 61.04      
Weighted Average Grant Date Fair Value, granted (in dollars per share) 90.17      
Weighted Average Grant Date Fair Value, at end of the period (in dollars per share) $ 61.04 $ 61.04   $ 69.61