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Receivables (Tables)
 12 Months Ended
Dec. 31, 2014
Receivables [Abstract]  
Allowance for bad debts
Millions of dollars
Balance at Beginning of Period
Charged to Costs and Expenses
Write-Offs
Balance at End of Period
Year ended December 31, 2012
$
137

$
(40
)
$
(5
)
$
92

Year ended December 31, 2013
92

39

(14
)
117

Year ended December 31, 2014
117

26

(6
)
137