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Acquisitions and Dispositions - Acquisitions (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Oct. 03, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Business Acquisition [Line Items]          
Asset Impairment Charges   $ 207 $ 10 $ 216 $ 14
Piedmont Natural Gas [Member]          
Business Acquisition [Line Items]          
Payments to acquire businesses $ 5,000        
Long-term debt assumed $ 2,002        
Unrealized Losses on Forward-Starting Interest Rate Swaps     22   190
Asset Impairment Charges   $ 7      
Business Combination, After Tax Integration Related Costs     $ 41   $ 161