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Acquisitions and Dispositions (Purchase Price Allocation) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Oct. 03, 2016
Business Acquisition [Line Items]      
Goodwill $ 19,396 $ 19,425  
Piedmont Natural Gas [Member]      
Business Acquisition [Line Items]      
Current assets     $ 497
Net property, plant and equipment     4,714
Goodwill     3,353
Other long-term assets     804
Total assets     9,368
Current liabilities, including current maturities of long-term debt     576
Long-term liabilities     1,790
Long-term debt     2,002
Total liabilities     4,368
Total purchase price     $ 5,000