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Employees, Retirement and Incentive Plans Restricted Units (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding at January 1, 2016 (nonvested) 101,408    
Granted 88,899    
Vesting and transfer of full ownership to recipients (61,768)    
Forfeited (4,551)    
Outstanding at December 31, 2016 (nonvested) 123,988 101,408  
Weighted Average Grant-Date Fair Value      
Outstanding at January 1, 2016 (nonvested) $ 33.63    
Granted 32.16    
Vesting and transfer of full ownership to recipients 32.81 $ 34.16 $ 33.49
Forfeited 34.21    
Outstanding at December 31, 2016 (nonvested) $ 32.96 $ 33.63  
Weighted average remaining contractual term (years) 1 year 7 months    
Grant date fair value of vested units transferred to recipients $ 2.0 $ 2.5 $ 2.7
Total unrecognized compensation related to nonvested units $ 2.8    
Minimum [Member] | Restricted Stock [Member]      
Weighted Average Grant-Date Fair Value      
Award vesting period 1 year    
Minimum [Member] | Restricted Stock Units (RSUs) [Member]      
Weighted Average Grant-Date Fair Value      
Award vesting period 1 year    
Maximum [Member] | Restricted Stock [Member]      
Weighted Average Grant-Date Fair Value      
Award vesting period 3 years    
Maximum [Member] | Restricted Stock Units (RSUs) [Member]      
Weighted Average Grant-Date Fair Value      
Award vesting period 3 years