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Share and Unit-Based Plans: SARs Narrative (Details) - $ / shares
 12 Months Ended
Dec. 08, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 09, 2015
Dec. 31, 2012
Share and unit-based plans            
Distributions paid, per share (in dollars per share) $ 2.00 $ 4.63 $ 2.51 $ 2.36    
SARs            
Share and unit-based plans            
Term of award (in years)   10 years        
Grant date of award (in dollars per share)   $ 9.67        
Volatility rate (as a percent)   25.85%        
Dividend yield (as a percent)   3.69%        
Risk free rate (as a percent)   1.20%        
Current value (in dollars per share)   $ 59.57        
Expected term (in years)   8 years        
Stock appreciation units balance (shares) 407,823 417,783 772,639 1,070,991 417,783 1,164,185
Stock appreciation weighted average price (in dollars per share) $ 56.49 $ 55.13 $ 56.67 $ 56.66 $ 55.13 $ 56.66
SARS granted prior to 2012            
Share and unit-based plans            
Grant date of award (in dollars per share)   $ 7.68        
Volatility rate (as a percent)   22.52%        
Dividend yield (as a percent)   5.23%        
Risk free rate (as a percent)   3.15%        
Current value (in dollars per share)   $ 61.17        
Expected term (in years)   8 years