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Rate and Regulatory Matters (Schedule of Regulatory Assets) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Regulatory Assets [Line Items]    
Regulatory Assets $ 58,552 $ 66,216
Retirement Benefits [Member]    
Regulatory Assets [Line Items]    
Regulatory Assets $ 39,378 47,868
Remaining recovery period
Various
  
Income Taxes [Member]    
Regulatory Assets [Line Items]    
Regulatory Assets $ 15,541 17,195
Remaining recovery period
Various
  
Rate Cases, Tank Painting, and Other [Member]    
Regulatory Assets [Line Items]    
Regulatory Assets $ 3,633 $ 1,153
Remaining recovery period 2-9 years