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Basis of Presentation and Summary of Significant Accounting Policies - New accounting policy (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                  
Operating income (loss) $ 78     $ (66)   $ 37 $ 20    
Other non operating income, net $ 43     50   $ 45 $ 58    
ASU 2017-07                  
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                  
Operating income (loss)   $ (5) $ (7) 9 $ 2     $ (1) $ 24
Other non operating income, net   $ 5 $ 7 $ (9) $ (2)     $ 1 $ (24)