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Allowance for Doubtful Accounts
 12 Months Ended
Dec. 31, 2017
Allowance for Doubtful Accounts [Abstract]  
Allowance for Doubtful Accounts
14. Allowance for Doubtful Accounts

Activity in the allowance for doubtful accounts is summarized as follows for the years presented (in thousands):

 
Year Ended December 31,
 
2017
 
2016
 
2015
Balance, beginning of year
$
1,277
 
$
749
 
$
911
Charges to expense
 
919
  
1,251
  
455
Uncollected balances written off, net of recoveries
 
(924)
  
(723)
  
(617)
Balance, end of year
$
1,272
 
$
1,277
 
$
749