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Share and Unit-based Plans - Stock Awards and Stock Units Roll Forward Activity (Details)
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Stock awards      
Shares or Units      
Balance at beginning of year (shares) | shares 0 0 1,612
Granted (shares) | shares 0 0 0
Vested (shares) | shares 0 0 (1,612)
Balance at end of year (shares) | shares 0 0 0
Weighted Average Grant Date Fair Value      
Balance at beginning of year (in dollars per share) | $ / shares $ 0.00 $ 0.00 $ 62.01
Granted (in dollars per share) | $ / shares 0.00 0.00 0.00
Vested (in dollars per share) | $ / shares 0.00 0.00 62.01
Balance at end of year (in dollars per share) | $ / shares $ 0.00 $ 0.00 $ 0.00
Stock units      
Shares or Units      
Balance at beginning of year (shares) | shares 151,355 148,428 132,086
Granted (shares) | shares 87,983 86,827 85,601
Vested (shares) | shares (108,991) (81,205) (69,259)
Forfeited (shares) | shares (890) (2,695) 0
Balance at end of year (shares) | shares 129,457 151,355 148,428
Weighted Average Grant Date Fair Value      
Balance at beginning of year (in dollars per share) | $ / shares $ 73.32 $ 78.53 $ 74.58
Granted (in dollars per share) | $ / shares 58.79 66.46 79.22
Vested (in dollars per share) | $ / shares 74.04 75.62 71.82
Forfeited (in dollars per share) | $ / shares 68.81 69.57 0.00
Balance at end of year (in dollars per share) | $ / shares $ 64.21 $ 73.32 $ 78.53
Number of common shares into which units can be converted (shares) 1