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Valuation and Qualifying Accounts
 12 Months Ended
Dec. 31, 2016
Valuation and Qualifying Accounts [Abstract]  
Valuation and Qualifying Accounts Valuation and Qualifying AccountsThe table below provides changes in the allowance for doubtful accounts over each period.
(in millions)
2016
 
2015
 
2014
Balance at beginning of year
$
22

 
$
24

 
$
22

Additions charged to expense
4

 
4

 
9

Deductions/other
(5
)
 
(6
)
 
(7
)
Balance at end of year
$
21

 
$
22

 
$
24