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Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2012
Accounting Policies [Abstract]  
Changes in the allowance for doubtful accounts receivable
The following table summarizes the changes in the allowance for doubtful accounts receivable for the indicated periods:
 
 
Balance at beginning of year
 
Additions charged to earnings
 
Uncollectible receivables written off, net of recoveries
 
Balance at
end of year
 
 
 
 
 
 
 
 
 
December 31, 2012
 
$
3,698

 
$
637

 
$
(1,772
)
 
$
2,563

December 31, 2011
 
5,339

 
428

 
(2,069
)
 
3,698

December 31, 2010
 
6,887

 
2,274

 
(3,822
)
 
5,339