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Stock Based Compensation - Rollforward of Stock Awards (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock Awards      
Rollforward of deferred stock award activity      
Unvested and Potential Stock Awards as of the beginning of the year (in shares) 1,045,020    
Unvested and Potential Stock Awards as of end of the year (in shares) 1,009,530 1,045,020  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Unvested and Potential Stock Awards, Weighted-Average Grant Date Fair Value at beginning of year (in dollars per share) $ 25.15    
Unvested and Potential Stock Awards, Weighted-Average Grant Date Fair Value at end of year (in dollars per share) $ 24.37 $ 25.15  
Deferred Stock Award      
Rollforward of deferred stock award activity      
Deferred Stock Awards Granted (in shares) 242,586    
Deferred Stock Awards Vested (in shares) (229,274)    
Deferred Stock Awards Forfeited (in shares) (6,979)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Deferred Stock Awards Granted, Weighted-Average Grant Date Fair (in dollars per share) $ 22.39 $ 21.02 $ 17.84
Deferred Stock Awards Vested, Weighted-Average Grant Date Fair Value (in dollars per share) 21.05    
Deferred Stock Awards Forfeited, Weighted Average Grant Date Fair Value (in dollars per share) $ 21.82    
Performance Shares      
Rollforward of deferred stock award activity      
Increase in Estimated Potential Future Performance Share Awards, net of forfeitures (in shares) 111,545    
Deferred Stock Awards Vested (in shares) (153,368)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Increase in Estimated Potential Future Performance Share Awards, Weighted Average Grant Date Fair Value (in dollars per share) $ 26.97    
Deferred Stock Awards Vested, Weighted-Average Grant Date Fair Value (in dollars per share) $ 30.45