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Consolidated Balance Sheet Parenthetical - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Allowance for doubtful accounts $ 11 $ 13
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized 400,000,000 400,000,000
Common stock, shares outstanding 131,636,870 140,227,692
Preferred stock, par value $ 0.01 $ 0.01
Preferred stock, shares authorized 50,000,000 50,000,000
Preferred stock, shares outstanding 0 0