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Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Sep. 30, 2015
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill $ 6,251 $ 6,251   $ 6,251 $ 2    
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 957 957   957      
Accumulated Amortization (62) (62)   (62) (12)    
Other Intangible Assets, Net 895 895   895      
Total other intangible assets, gross 1,032 1,032   1,032      
Total other intangible assets, net 970 970   970 317    
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
2017 108 108   108      
2018 93 93   93      
2019 74 74   74      
2020 63 63   63      
2021 56 56   56      
Amortization of intangible assets       50 3 $ 3  
Customer relationships              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 268 268   268      
Accumulated Amortization (32) (32)   (32)      
Other Intangible Assets, Net 236 236   236      
Trade names              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 158 158   158      
Accumulated Amortization (5) (5)   (5)      
Other Intangible Assets, Net 153 153   153      
Patents              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 4 4   4      
Accumulated Amortization 0 0   0      
Other Intangible Assets, Net 4 4   4      
Backlog              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 5 5   5      
Accumulated Amortization (1) (1)   (1)      
Other Intangible Assets, Net 4 4   4      
Storage and transportation contracts              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 64 64   64      
Accumulated Amortization (2) (2)   (2)      
Other Intangible Assets, Net 62 62   62      
Software and other              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 2 2   2      
Accumulated Amortization 0 0   0      
Other Intangible Assets, Net 2 2   2      
PPA fair value adjustments              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 456 456   456      
Accumulated Amortization (22) (22)   (22)      
Other Intangible Assets, Net 434 434   $ 434      
Minimum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       11 years      
Minimum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       5 years      
Minimum [Member] | Patents              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       3 years      
Minimum [Member] | Storage and transportation contracts              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       1 year      
Minimum [Member] | Software and other              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       1 year      
Minimum [Member] | PPA fair value adjustments              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       19 years      
Maximum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       26 years      
Maximum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       28 years      
Maximum [Member] | Patents              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       10 years      
Maximum [Member] | Storage and transportation contracts              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       5 years      
Maximum [Member] | Software and other              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       12 years      
Maximum [Member] | PPA fair value adjustments              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       20 years      
Southern Company Gas [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill impairment testing fair value in excess of carrying value percent           5.00%  
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
2017 73 73   $ 73      
2018 58 58   58      
2019 40 40   40      
2020 28 28   28      
2021 21 21   21      
Southern Company Gas [Member] | Predecessor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill         1,813    
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount         177    
Accumulated Amortization         (68)    
Other Intangible Assets, Net         109    
Total other intangible assets, net         109    
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
Amortization of intangible assets     $ 8   18 $ 20  
Southern Company Gas [Member] | Predecessor [Member] | Customer relationships              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount         132    
Accumulated Amortization         (57)    
Other Intangible Assets, Net         75    
Southern Company Gas [Member] | Predecessor [Member] | Trade names              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount         45    
Accumulated Amortization         (11)    
Other Intangible Assets, Net         $ 34    
Southern Company Gas [Member] | Predecessor [Member] | Minimum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life         11 years    
Southern Company Gas [Member] | Predecessor [Member] | Minimum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life         10 years    
Southern Company Gas [Member] | Predecessor [Member] | Maximum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life         14 years    
Southern Company Gas [Member] | Predecessor [Member] | Maximum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life         28 years    
Southern Company Gas [Member] | Successor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill 5,967 5,967   5,967      
Goodwill 30            
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 400 400   400      
Accumulated Amortization (34) (34)   (34)      
Other Intangible Assets, Net 366 366   366      
Total other intangible assets, net 366 366   366      
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
Amortization of intangible assets   32          
Southern Company Gas [Member] | Successor [Member] | Customer relationships              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 221 221   221      
Accumulated Amortization (30) (30)   (30)      
Other Intangible Assets, Net 191 191   191      
Southern Company Gas [Member] | Successor [Member] | Trade names              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 115 115   115      
Accumulated Amortization (2) (2)   (2)      
Other Intangible Assets, Net 113 113   113      
Southern Company Gas [Member] | Successor [Member] | Storage and transportation contracts              
Finite-Lived Intangible Assets, Net [Abstract]              
Gross Carrying Amount 64 64   64      
Accumulated Amortization (2) (2)   (2)      
Other Intangible Assets, Net 62 62   $ 62      
Southern Company Gas [Member] | Successor [Member] | Minimum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       11 years      
Southern Company Gas [Member] | Successor [Member] | Minimum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       10 years      
Southern Company Gas [Member] | Successor [Member] | Minimum [Member] | Storage and transportation contracts              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       1 year      
Southern Company Gas [Member] | Successor [Member] | Maximum [Member] | Customer relationships              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       14 years      
Southern Company Gas [Member] | Successor [Member] | Maximum [Member] | Trade names              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       28 years      
Southern Company Gas [Member] | Successor [Member] | Maximum [Member] | Storage and transportation contracts              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       5 years      
Southern Company [Member] | Backlog              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       5 years      
Southern Company [Member] | FCC Licenses [Member]              
Finite-Lived Intangible Assets, Net [Abstract]              
Other intangible assets not subject to amortization: 75 75   $ 75      
Southern Power [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Finite-Lived Intangible Asset, Useful Life       19 years      
Finite-Lived Intangible Assets, Net [Abstract]              
Accumulated Amortization (22) (22)   $ (22) $ (12)    
Total other intangible assets, net         317    
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
2017 25 25   25      
2018 25 25   25      
2019 25 25   25      
2020 25 25   25      
2021 25 25   25      
Amortization of intangible assets       10 3 3  
Gas Distribution Operations [Member] | Southern Company Gas [Member] | Predecessor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill         1,640    
Gas Distribution Operations [Member] | Southern Company Gas [Member] | Successor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill 4,702 4,702   4,702      
Gas Marketing Services [Member] | Southern Company Gas [Member] | Predecessor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill         $ 173    
Gas Marketing Services [Member] | Southern Company Gas [Member] | Successor [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill $ 1,265 $ 1,265   1,265      
Storage and Fuels Reporting Unit [Member] | Southern Company Gas [Member]              
Acquired Finite-Lived Intangible Assets [Line Items]              
Goodwill           $ 14 $ 14
Wholesale Gas Services [Member] | Southern Company Gas [Member] | Successor [Member]              
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]              
Amortization of intangible assets       $ 2