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Summary of Significant Accounting Policies (Schedule of Trade Receivable) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Summary of trade receivables, net        
Trade receivables $ 1,687 $ 1,591    
Allowance for doubtful accounts (63) (41) $ (16) $ (16)
Total trade receivables, net $ 1,624 $ 1,550