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Allowance for Uncollectible Accounts
 12 Months Ended
Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]  
Allowance for Uncollectible Accounts
Allowance for Uncollectible Accounts
The following table summarizes the changes in the Company’s allowances for uncollectible accounts for the years ended December 31:
 
2017
 
2016
 
2015
Balance as of January 1
$
(40
)
 
$
(39
)
 
$
(35
)
Amounts charged to expense
(29
)
 
(27
)
 
(32
)
Amounts written off
30

 
29

 
31

Recoveries of amounts written off
(3
)
 
(3
)
 
(3
)
Balance as of December 31
$
(42
)
 
$
(40
)
 
$
(39
)