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Acquisitions and Divestitures Sale of Equity Interest in SNG (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 01, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from sale of equity interests in subsidiaries, net   $ 0 $ 1,401 $ 0
Bear Creek Storage        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Equity Method Investment, Ownership Percentage   75.00%    
Sale Equity Interest in SNG [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Disposal Group, Equity Interest Sold 50.00%      
Proceeds from sale of equity interests in subsidiaries, net $ 1,400      
Gain (loss) on impairments and divestitures, net     $ (84)  
Sale Equity Interest in SNG [Member] | Southern Natural Gas Company LLC        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Disposal Group, Equity Interest Sold 50.00%      
Equity Method Investment, Ownership Percentage 50.00%      
Sale Equity Interest in SNG [Member] | Bear Creek Storage        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Equity Method Investment, Ownership Percentage 50.00%