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Accounts Receivable and Unbilled Services (Tables)
 9 Months Ended
Sep. 30, 2017
Receivables [Abstract]  
Accounts Receivable and Unbilled Services

Accounts receivable and unbilled services consist of the following:

 

(in millions)

 

September 30, 2017

 

 

December 31, 2016

 

Trade:

 

 

 

 

 

 

 

 

Billed

 

$

1,044

 

 

$

998

 

Unbilled services

 

 

828

 

 

 

723

 

 

 

 

1,872

 

 

 

1,721

 

Allowance for doubtful accounts

 

 

(13

)

 

 

(14

)

 

 

$

1,859

 

 

$

1,707