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Share and Unit-based Plans - Directors' Phantom Stock Plan (Details)
 12 Months Ended
Dec. 31, 2018
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Phantom stock units      
Share and unit-based plans      
Number of common shares into which units can be converted (shares) 1    
Units      
Balance at beginning of year (shares) 4,054 5,845 0
Granted (shares) 10,380 8,760 21,088
Vested (shares) (12,193) (10,551) (15,243)
Forfeited (shares) (2,241) 0 0
Balance at end of year (shares) 0 4,054 5,845
Weighted Average Grant Date Fair Value (in dollars per share)      
Balance at beginning of year (in dollars per share) | $ / shares $ 79.82 $ 81.47 $ 0.00
Granted (in dollars per share) | $ / shares 49.55 68.93 80.21
Vested (in dollars per share) | $ / shares 54.40 71.69 79.73
Forfeited (in dollars per share) | $ / shares 77.91 0.00 0.00
Balance at end of year (in dollars per share) | $ / shares $ 0.00 $ 79.82 $ 81.47
Director's Phantom Stock Plan      
Share and unit-based plans      
Deferral period for grant of units (in years) 3 years    
Number of common shares into which units can be converted (shares) 1    
Maximum shares authorized under plan (shares) 500,000    
Shares available for issuance under plan (in shares) 159,374