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Receivables (Tables)
 12 Months Ended
Dec. 31, 2017
Transfers and Servicing [Abstract]  
Schedule of Allowance for Doubtful Accounts
The following table details the changes in the allowance for doubtful accounts related to customer receivables and other receivables:
 
 
Customer receivables
 
Other receivables
December 31,
2017
2016
2015
 
2017
2016
2015
Balance at beginning of year
$
13

$
8

$
6

 
$
32

$
34

$
24

Provision for doubtful accounts
1

7

4

 
9

6

8

Write off of uncollectible accounts
(5
)
(3
)
(2
)
 
(1
)
(1
)
2

Recoveries of prior write-offs
—

(1
)
—

 
(3
)
1

(1
)
Other
(1
)
2

—

 
(3
)
(8
)
1

Balance at end of year
$
8

$
13

$
8

 
$
34

$
32

$
34