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CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)
$ in Millions
Jun. 30, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Materials and supplies, allowance for obsolescence | $ $ 150 $ 153
Shares, authorized 417,060,033 417,060,033
Shares, conditionally authorized 143,783,041 143,783,041
Shares, issued 394,801,990 394,801,990
Shares, outstanding 391,181,430 389,366,241
Maximum    
Allowance for doubtful accounts | $ $ 1 $ 1