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Fair Value of Financial Assets and Liabilities (Tables)
6 Months Ended
Jun. 30, 2012
Fair Value of Financial Assets and Liabilities [Abstract]  
Carrying Amount and Fair Value of Long-term Debt
As of June 30, 2012 and Dec. 31, 2011, other financial instruments for which the carrying amount did not equal fair value were as follows:
 
   
June 30, 2012
  
Dec. 31, 2011
 
(Thousands of Dollars)
 
Carrying
Amount
  
Fair Value
  
Carrying
Amount
  
Fair Value
 
Long-term debt, including current portion
 $1,103,566  $1,306,907  $993,314  $1,176,020