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Summary of Significant Accounting Policies - Affiliate Transactions (Details)
1 Months Ended 4 Months Ended 6 Months Ended 12 Months Ended
Oct. 04, 2016
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2017
USD ($)
location
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2016
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       $ 880,000,000 $ 880,000,000   $ 691,000,000 $ 880,000,000    
Purchased power             863,000,000 750,000,000 $ 645,000,000  
Securities due within one year       2,587,000,000 2,587,000,000   3,892,000,000 2,587,000,000    
ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       63,000,000 63,000,000   63,000,000 63,000,000    
Cost of Purchased Power Affiliates             $ 158,000,000 168,000,000 180,000,000  
Ownership percentage, equity method investment             50.00%      
Capital contributions from parent company             $ 361,000,000 260,000,000 22,000,000  
Securities due within one year       561,000,000 561,000,000   0 561,000,000    
MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       26,000,000 26,000,000   33,000,000 26,000,000    
Cost of Purchased Power Affiliates             25,000,000 34,000,000 12,000,000  
Capital contributions from parent company     $ 1,000,000,000       1,002,000,000 627,000,000 277,000,000  
Issuance of promissory note to parent related to repayment of interest-bearing refundable deposits and accrued interest             0 0 301,000,000  
Securities due within one year       629,000,000 629,000,000   989,000,000 629,000,000    
Long-term debt affiliated       551,000,000 551,000,000   0 551,000,000    
SOUTHERN Co GAS                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       127,000,000 127,000,000   88,000,000 127,000,000    
Intangible liabilities, accumulated amortization             50,000,000      
Capital contributions from parent company         1,085,000,000   103,000,000      
Securities due within one year       22,000,000 22,000,000   157,000,000 22,000,000    
GULF POWER CO                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       71,000,000 71,000,000   67,000,000 71,000,000    
Cost of Purchased Power Affiliates             15,000,000 16,000,000 35,000,000  
Purchased power             155,000,000 142,000,000 135,000,000  
Capital contributions from parent company             2,000,000 20,000,000 4,000,000  
Securities due within one year       87,000,000 87,000,000   0 87,000,000    
SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       368,000,000 368,000,000   322,000,000 368,000,000    
Deferred project development costs       11,000,000 11,000,000     11,000,000    
Purchased power             27,000,000 21,000,000 21,000,000  
Capital contributions from parent company             0 1,850,000,000 646,000,000  
Securities due within one year       560,000,000 560,000,000   770,000,000 560,000,000    
Repayments of debt   $ 40,000,000                
GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Other liabilities noncurrent       199,000,000 199,000,000   $ 128,000,000 199,000,000    
Number of military bases for renewable generation | location             2      
Cost of Purchased Power Affiliates             $ 622,000,000 518,000,000 575,000,000  
Capital contributions from parent company             431,000,000 594,000,000 62,000,000  
Securities due within one year       460,000,000 460,000,000   857,000,000 460,000,000    
PowerSecure International, Inc. | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             11,000,000      
PowerSecure International, Inc. | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount $ 119,000,000                  
Southern Natural Gas Company, LLC | Southern Company Services, Inc.                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       32,000,000     136,000,000      
Cost of natural gas purchases           $ 27,000,000 142,000,000      
Southern Natural Gas Company, LLC | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       2,000,000     9,000,000      
Southern Natural Gas Company, LLC | SOUTHERN Co GAS                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       15,000,000     32,000,000      
Southern Natural Gas Company, LLC | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       7,000,000     25,000,000      
Southern Natural Gas Company, LLC | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       35,000,000     102,000,000      
Southern Company Services, Inc. | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             479,000,000 460,000,000 438,000,000  
Southern Company Services, Inc. | MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             140,000,000 231,000,000 295,000,000  
Southern Company Services, Inc. | SOUTHERN Co GAS                    
Related Party Transaction [Line Items]                    
Related party transaction, amount         17,000,000   63,000,000      
Southern Company Services, Inc. | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             81,000,000 80,000,000 81,000,000  
Southern Company Services, Inc. | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             218,000,000 193,000,000 146,000,000  
Southern Company Services, Inc. | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             625,000,000 606,000,000 585,000,000  
GEORGIA POWER CO | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             11,000,000 8,000,000 12,000,000  
MISSISSIPPI POWER CO | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             31,000,000 26,000,000 27,000,000  
ALABAMA POWER CO | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Revenue requirements reimbursement             11,000,000 12,000,000 14,000,000  
GULF POWER CO | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Revenue requirements reimbursement             11,000,000 12,000,000 14,000,000  
GULF POWER CO | MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             $ 31,000,000 26,000,000 27,000,000  
GULF POWER CO | GEORGIA POWER CO | Plant Scherer Unit 3 (coal)                    
Related Party Transaction [Line Items]                    
Long-term purchase commitment, reimbursement percentage             25.00%      
SOUTHERN Co GAS | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount         $ 17,000,000   $ 119,000,000      
SOUTHERN Co GAS | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount       $ 10,000,000     22,000,000      
SOUTHERN POWER CO | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             6,800,000      
SOUTHERN POWER CO | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             235,000,000 265,000,000 179,000,000  
Southern Nuclear Operating Company, Inc. | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             248,000,000 249,000,000 243,000,000  
Southern Nuclear Operating Company, Inc. | GEORGIA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             675,000,000 666,000,000 681,000,000  
Scenario, Plan | ALABAMA POWER CO | GULF POWER CO                    
Related Party Transaction [Line Items]                    
Revenue requirements reimbursement             10,000,000      
Scenario, Plan | GULF POWER CO | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Revenue requirements reimbursement             61,000,000      
Southern Natural Gas Company, LLC | SOUTHERN Co GAS                    
Related Party Transaction [Line Items]                    
Ownership percentage, equity method investment                   50.00%
Non-Fuel Expense | MISSISSIPPI POWER CO | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             9,000,000 13,000,000 11,000,000  
Non-Fuel Expense | ALABAMA POWER CO | MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             9,000,000 13,000,000 11,000,000  
Non-Fuel Expense | GULF POWER CO | GEORGIA POWER CO | Plant Scherer Unit 3 (coal)                    
Related Party Transaction [Line Items]                    
Reimbursement Revenue             11,000,000 8,000,000 12,000,000  
Fuel Purchases | MISSISSIPPI POWER CO | ALABAMA POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             0 0 8,000,000  
Fuel Purchases | ALABAMA POWER CO | MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount                 8,000,000  
Purchased Power from Affiliates | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             233,000,000 258,000,000 219,000,000  
Operating Lease PPA | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             81,000,000 109,000,000 109,000,000  
Operations and Maintenance Expense | Southern Company Services, Inc. | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             192,000,000 173,000,000 138,000,000  
Electric Transmission | Southern Company Services, Inc. | SOUTHERN POWER CO                    
Related Party Transaction [Line Items]                    
Related party transaction, amount             $ 13,000,000 $ 11,000,000 $ 11,000,000  
Promissory Notes | Promissory Notes | MISSISSIPPI POWER CO                    
Related Party Transaction [Line Items]                    
Repayments of debt   109,000,000 $ 591,000,000              
Debt instrument, face amount   $ 150,000,000