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Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 12 Months Ended
Feb. 28, 2019
Jan. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Performance Shares [Member]          
Stock-based Compensation [Line Items]          
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition     1 year 8 months    
Number of Shares [Rollforward]          
As of January 1   129,693 127,898 127,580 119,540
Granted [1]     66,557 50,729 57,189
Awarded     (58,293) 0 0
Unearned Grant Award     0 (40,801) (42,126)
Forfeited     (6,469) (9,610) (7,023)
As of December 31     129,693 127,898 127,580
Performance Period (Years)     3 years    
Weighted-Average Grant Date Fair Value [Abstract]          
As of January 1   $ 66.12 $ 58.23 $ 52.56 $ 52.72
Granted [1]     76.42 62.90 52.43
Awarded     59.82 0.00 0.00
Unearned Grant Award     0.00 46.27 52.70
Forfeited     72.99 58.29 53.45
As of December 31     $ 66.12 $ 58.23 $ 52.56
Restricted Stock Units [Member]          
Stock-based Compensation [Line Items]          
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition     1 year 7 months    
Number of Shares [Rollforward]          
As of January 1   49,771 55,248 54,728 57,694
Granted [2]     16,573 21,241 20,351
Awarded     (18,881) (17,281) (19,661)
Forfeited     (3,169) (3,440) (3,656)
As of December 31     49,771 55,248 54,728
Performance Period (Years)     3 years    
Weighted-Average Grant Date Fair Value [Abstract]          
As of January 1   $ 60.74 $ 56.18 $ 51.79 $ 49.86
Granted [2]     71.11 62.20 50.25
Awarded     55.78 49.72 44.33
Forfeited     64.92 56.00 52.87
As of December 31     $ 60.74 $ 56.18 $ 51.79
Subsequent Event [Member] | Performance Shares [Member]          
Number of Shares [Rollforward]          
Granted   31,843      
Awarded (73,532)        
Performance Period (Years)   3 years      
Granted, Grant Date Fair Value   $ 2.3      
Awarded, Grant Date Fair Value $ 3.9        
Subsequent Event [Member] | Restricted Stock Units [Member]          
Number of Shares [Rollforward]          
Granted   12,924      
Awarded (14,307)        
Granted, Grant Date Fair Value   $ 1.0      
Awarded, Grant Date Fair Value $ 0.7        
[1] Shares granted include accrued dividends.
[2] Shares granted include accrued dividends.