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Rate and Regulatory Matters (Tables)
 12 Months Ended
Dec. 31, 2017
Regulated Operations [Abstract]  
Schedule of regulatory assets

We have recorded certain costs as regulatory assets because we expect full recovery of, or are currently recovering, these costs in the rates we charge customers. These deferred costs have been excluded from rate base and, therefore, we are not earning a return on the unamortized balances. These items are detailed as follows:

 

   (Thousands of Dollars)    
   December 31,   Remaining
   Regulatory Assets  2017   2016   Recovery Periods
Retirement Benefits  $43,070   $40,603   Various
Income Taxes   9,876    15,899   Various
Rate Cases, Tank Painting, and Other   5,477    4,392   2-10 years
Total  $58,423   $60,894