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Allowance for Doubtful Accounts (Tables)
 12 Months Ended
Dec. 31, 2017
Receivables [Abstract]  
Allowance for Doubtful Accounts Roll Forward
Activity in the allowance for doubtful accounts was as follows:
 
 
Years ended December 31,
 
 
2017
 
2016
 
2015
Balance, beginning of year
 
$
1,829

 
$
1,417

 
$
1,844

Provision (net of recovery)
 
370

 
898

 
346

Amounts written off, net of recoveries
 
(242
)
 
(486
)
 
(773
)
Balance, end of year
 
$
1,957

 
$
1,829

 
$
1,417