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Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]  
Summary of Changes in the Allowance for Doubtful Accounts

The following table presents the changes in our allowance for doubtful accounts for the periods presented:

 

 

 

December 31,

 

 

 

2018

 

 

2017

 

 

2016

 

 

 

(in millions)

 

Allowance for doubtful accounts:

 

 

 

 

 

 

 

 

 

 

 

 

Balance, beginning of period

 

$

16

 

 

$

9

 

 

$

6

 

Charges to earnings

 

 

11

 

 

 

8

 

 

 

4

 

Write-offs, net of recoveries and other

   adjustments

 

 

(6

)

 

 

(1

)

 

 

(1

)

Balance, end of period

 

$

21

 

 

$

16

 

 

$

9