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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 $ 1,713 $ 1,680    
Allowance for doubtful accounts (63) (41) $ (16) $ (16)
Total trade receivables, net 1,650 1,639    
Billed Revenues        
Summary of trade receivables, net        
Trade receivables 1,479 1,452    
Unbilled Revenues        
Summary of trade receivables, net        
Trade receivables $ 234 $ 228