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Acquisitions and Dispositions - Acquisitions (Narrative) (Details) - Piedmont Natural Gas [Member] - USD ($)
$ in Millions
3 Months Ended
Oct. 03, 2016
Mar. 31, 2017
Mar. 31, 2016
Business Acquisition [Line Items]      
Payments to acquire businesses $ 5,000    
Long-term debt assumed $ 2,002    
Business Combination, Integration Related Costs   $ 16 $ 101
Unrealized Losses on Forward-Starting Interest Rate Swaps     93
Business Combination, After Tax Integration Related Costs     63
Interest Expense [Member]      
Business Acquisition [Line Items]      
Business Combination, Integration Related Costs     $ 100