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Accounts Receivable (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Accounts Receivable [Line Items]    
Unbillable, expected to be collected after one year $ 1,564 $ 2,011
Commercial Customers [Member]    
Accounts Receivable [Line Items]    
Unbillable, expected to be collected after one year $ 172 $ 172