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Fair Value of Financial Instruments Fair value by balance sheet grouping table (Tables)
 3 Months Ended
Mar. 31, 2016
Fair Value Disclosures [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments
The carrying amount and fair value of our long-term debt, including current maturities, is as follows:
 
March 31, 2016
 
December 31, 2015
Millions of dollars
Level 1
Level 2
Total fair value
Carrying value
 
Level 1
Level 2
Total fair value
Carrying value
Long-term debt
$
1,311

$
14,819

$
16,130

$
15,393

 
$
1,009

$
14,947

$
15,956

$
15,346