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Share and Unit-Based Plans: Directors' Phantom Stock Plan (Details)
 12 Months Ended
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
$ / shares
shares
Dec. 31, 2013
$ / shares
shares
Phantom stock units      
Share and unit-based plans      
Number of common shares into which units can be converted (in shares) 1    
Units      
Balance at beginning of year 9,269 17,575 0
Granted 13,351 10,747 34,266
Vested (20,162) (19,053) (16,691)
Forfeited (2,458) 0 0
Balance at end of year 0 9,269 17,575
Weighted Average Grant Date Fair Value (in dollars per share)      
Balance at beginning of year | $ / shares $ 58.35 $ 58.66 $ 0.00
Granted | $ / shares 78.72 65.54 59.04
Vested | $ / shares 72.17 62.69 59.44
Forfeited | $ / shares 55.62 0.00 0.00
Balance at end of year | $ / shares $ 0.00 $ 58.35 $ 58.66
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 (in shares) 1    
Maximum shares authorized under plan (in shares) 500,000    
Shares available for issuance under plan (in shares) 199,603