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Share and Unit-Based Plans - Directors' Phantom Stock Plan (Details)
 12 Months Ended
Dec. 31, 2016
$ / shares
shares
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
$ / 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) 0 9,269 17,575
Granted (shares) 21,088 13,351 10,747
Vested (shares) (15,243) (20,162) (19,053)
Forfeited (shares) 0 (2,458) 0
Balance at end of year (shares) 5,845 0 9,269
Weighted Average Grant Date Fair Value (in dollars per share)      
Balance at beginning of year (in dollars per share) | $ / shares $ 0.00 $ 58.35 $ 58.66
Granted (in dollars per share) | $ / shares 80.21 78.72 65.54
Vested (in dollars per share) | $ / shares 79.73 72.17 62.69
Forfeited (in dollars per share) | $ / shares 0.00 55.62 0.00
Balance at end of year (in dollars per share) | $ / shares $ 81.47 $ 0.00 $ 58.35
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) 178,515