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Consolidated Balance Sheets (Parenthetical) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Statement of Financial Position [Abstract]    
Allowance for doubtful accounts $ 10 $ 11
Preferred stock:    
Shares issued 0 0
Shares outstanding 0 0
Par value $ 0.01  
Shares authorized 30,000,000  
Common stock:    
Shares issued 735,000,000 734,000,000
Par value $ 0.01  
Shares authorized 1,000,000,000  
Treasury stock (284,000,000) (248,000,000)