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Transactions With Major Customers and Affiliates (Tables)
12 Months Ended
Dec. 31, 2012
Transactions With Major Customers And Affiliates [Abstract]  
ScheduleOfRevenueByMajorCustomersByReportingSegmentsTableTextBlock
      Years Ended December 31,
      2012 2011 2010
              
      (Thousands of Dollars)
        
Puget Sound Energy, Inc. $ 100,799 $ 99,116 $ 95,564
Northwest Natural Gas Co.   47,297   47,322   48,022