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Acquisitions - Pro Forma Consolidated Information (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Mankato [Member]    
Business Acquisition [Line Items]    
Business Acquisition, Pro Forma Revenue $ 40 $ 39
Business Acquisition, Pro Forma Net Income (Loss) 14 11
Southern Company Gas [Member]    
Business Acquisition [Line Items]    
Business Acquisition, Pro Forma Revenue 21,791 21,430
Business Acquisition, Pro Forma Net Income (Loss) $ 2,591 $ 2,665
Business Acquisition, Pro Forma Earnings Per Share, Basic $ 2.70 $ 2.85
Business Acquisition, Pro Forma Earnings Per Share, Diluted $ 2.68 $ 2.84