v3.10.0.1
Regulatory Matters - Schedule of GAS Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 720 $ (1,894)
Other cost of removal obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (2,297) (2,684)
Deferred income tax credits    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (6,455) (7,261)
Over recovered regulatory clause revenues    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (47) (155)
Other regulatory liabilities    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (132) (104)
Retiree benefit plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 3,658 3,931
Retiree benefit plans | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 15 years  
Long-term debt fair value adjustment    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 121 138
Under recovered regulatory clause revenues    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets 407 317
Other regulatory assets    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 581 625
Other regulatory assets | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 50 years  
Southern Company Gas    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ (1,872) (1,879)
Southern Company Gas | Other cost of removal obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (1,585) (1,646)
Southern Company Gas | Deferred income tax credits    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (940) (1,063)
Southern Company Gas | Over recovered regulatory clause revenues    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (43) (144)
Southern Company Gas | Other regulatory liabilities    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets (46) (21)
Southern Company Gas | Environmental remediation    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets 311 410
Southern Company Gas | Retiree benefit plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 161 270
Southern Company Gas | Retiree benefit plans | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 15 years  
Southern Company Gas | Long-term debt fair value adjustment    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 121 138
Southern Company Gas | Long-term debt fair value adjustment | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 20 years  
Southern Company Gas | Under recovered regulatory clause revenues    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 90 98
Regulatory amortization period 7 years  
Southern Company Gas | Other regulatory assets    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Net regulatory assets $ 59 $ 79
Southern Company Gas | Other regulatory assets | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 10 years  
Southern Company Gas | Financial Instrument Hedging | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 2 years  
Southern Company Gas | Deferred Income Tax Charges and Other Cost of Removal Obligations | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 80 years  
Southern Company Gas | Other regulatory liabilities | Maximum    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory amortization period 20 years