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Receivables
 12 Months Ended
Dec. 31, 2012
Receivables [Abstract]  
Receivables
Receivables
Our trade receivables are generally not collateralized. At December 31, 2012 and December 31, 2011, 36% and 45% of our gross trade receivables were from customers in the United States. No other country or single customer accounted for more than 10% of our gross trade receivables at these dates.
The following table presents a rollforward of our allowance for bad debts for 2010, 2011, and 2012.

Millions of dollars
Balance at Beginning of Period
Charged to Costs and Expenses
Write-Offs
Balance at End of Period
Year ended December 31, 2010
$
90

$
5

$
(4
)
$
91

Year ended December 31, 2011
91

53

(7
)
137

Year ended December 31, 2012
137

(40)

(5
)
92