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Receivables (Tables)
 12 Months Ended
Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]  
Allowance for Credit Losses on Financing Receivables [Table Text Block]
The following table presents a rollforward of our allowance for bad debts for 2014, 2015 and 2016.
Millions of dollars
Balance at Beginning of Period
Charged to Costs and Expenses
Write-Offs
Balance at End of Period
Year ended December 31, 2014
$
117

$
26

$
(6
)
$
137

Year ended December 31, 2015
137

44

(36
)
145

Year ended December 31, 2016
145

50

(20
)
175