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Financial Instruments and Risk Management (Tables)
 12 Months Ended
Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments
:
 
December 31, 2017
 
December 31, 2016
Millions of dollars
Level 1
Level 2
Total fair value
Carrying value
 
Level 1
Level 2
Total fair value
Carrying value
Total debt
$
3,285

$
9,172

$
12,457

$
10,942

 
$
753

$
12,812

$
13,565

$
12,384