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Acquisition (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jan. 03, 2017
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]            
Goodwill   $ 867   $ 867   $ 862
Atmos Energy Marketing [Member]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]            
Total purchase price consideration $ 147          
Cash 15          
Receivables 140          
Natural gas inventory 78          
Derivative assets 35          
Prepaid expenses and other current assets 5          
Property and equipment 8          
Identifiable intangibles 25          
Total assets acquired 306          
Accounts payable 113          
Derivative liabilities 43          
Other current liabilities 7          
Other liabilities 1          
Total liabilities assumed 164          
Identifiable net assets acquired 142          
Goodwill 5          
Net assets acquired 147          
Amortization of intangible assets   1   1    
Business Acquisition, Pro Forma Information [Abstract]            
Revenue of acquiree since acquisition date   319   678    
Operating income of acquiree since acquisition date   8   25    
Operating Revenue   2,143 $ 1,773 4,878 $ 4,016  
Net Income   $ 135 $ 3 $ 327 $ 156  
Atmos Energy Marketing [Member] | Customer Relationships [Member]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]            
Intangibles estimated fair value $ 25          
Intangibles estimated useful life 15 years