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Retirement Benefits - Components of Accumulated OCI and Changes in Regulatory Assets (Details) - USD ($)
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity $ 26,612,000,000 $ 21,982,000,000 $ 25,528,000,000 $ 26,612,000,000 $ 21,982,000,000 $ 29,039,000,000 $ 25,528,000,000 $ 26,612,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance   21,982,000,000 25,528,000,000 26,612,000,000 21,982,000,000      
Total other comprehensive income (loss)     26,000,000 (9,000,000) (50,000,000)      
Ending balance 26,612,000,000   29,039,000,000 25,528,000,000 26,612,000,000      
Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity     (70,000,000) (70,000,000)   (82,000,000) (70,000,000)  
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     (70,000,000)          
Total other comprehensive income (loss)     2,000,000          
Ending balance     (82,000,000) (70,000,000)        
Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           (3,000,000) 3,000,000  
Net (gain) loss           100,000,000 104,000,000  
AOCI           97,000,000 107,000,000  
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     (9,000,000)          
Pension plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           17,000,000 14,000,000  
Net (gain) loss           3,441,000,000 3,140,000,000  
Regulatory amortization           0 0  
Regulatory asset           3,458,000,000 3,154,000,000  
Pension plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 100,000,000   107,000,000 100,000,000 100,000,000 97,000,000 107,000,000 100,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     107,000,000 100,000,000        
Reclassification adjustments       (8,000,000)        
Total other comprehensive income (loss)     (10,000,000) 7,000,000        
Ending balance 100,000,000   97,000,000 107,000,000 100,000,000      
Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 3,120,000,000   3,155,000,000 3,120,000,000 3,120,000,000 3,458,000,000 3,155,000,000 3,120,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     3,155,000,000 3,120,000,000        
Reclassification adjustments     (208,000,000) (166,000,000)        
Total other comprehensive income (loss)     303,000,000 35,000,000        
Ending balance 3,120,000,000   3,458,000,000 3,155,000,000 3,120,000,000      
Pension plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (7,000,000) (15,000,000)        
Reclassification adjustments     (9,000,000) (7,000,000)        
Pension plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (498,000,000) (227,000,000)        
Reclassification adjustments     (204,000,000) (155,000,000)        
Pension plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments       (1,000,000)        
Pension plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (1,000,000) 26,000,000        
Reclassification adjustments     (4,000,000) (11,000,000)        
Pension plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0 0        
Pension plans | Dispositions                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     8,000,000          
Pension plans | Dispositions | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (12,000,000)          
Pension plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       0        
Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           1,000,000 0  
Net (gain) loss           (5,000,000) 4,000,000  
AOCI           (4,000,000) 4,000,000  
Other postretirement benefit plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           14,000,000 21,000,000  
Net (gain) loss           8,000,000 320,000,000  
Regulatory amortization           0 0  
Regulatory asset           22,000,000 341,000,000  
Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 7,000,000   4,000,000 7,000,000 7,000,000 (4,000,000) 4,000,000 7,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     4,000,000 7,000,000        
Total other comprehensive income (loss)     (8,000,000) (3,000,000)        
Ending balance 7,000,000   (4,000,000) 4,000,000 7,000,000      
Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 378,000,000   341,000,000 378,000,000 378,000,000 22,000,000 341,000,000 378,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     341,000,000 378,000,000        
Reclassification adjustments     (21,000,000) (19,000,000)        
Total other comprehensive income (loss)     (319,000,000) (37,000,000)        
Ending balance 378,000,000   22,000,000 341,000,000 378,000,000      
Other postretirement benefit plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (8,000,000) (3,000,000)        
Other postretirement benefit plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     298,000,000 21,000,000        
Reclassification adjustments     (14,000,000) (13,000,000)        
Other postretirement benefit plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Other postretirement benefit plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 (3,000,000)        
Reclassification adjustments     (7,000,000) (6,000,000)        
Other postretirement benefit plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0          
Other postretirement benefit plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       0        
Alabama Power                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 6,323,000,000 5,992,000,000 6,829,000,000 6,323,000,000 5,992,000,000 7,477,000,000 6,829,000,000 6,323,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance   5,992,000,000 6,829,000,000 6,323,000,000 5,992,000,000      
Total other comprehensive income (loss)     4,000,000 4,000,000 2,000,000      
Ending balance 6,323,000,000   7,477,000,000 6,829,000,000 6,323,000,000      
Alabama Power | Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Alabama Power | Pension plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           6,000,000 8,000,000  
Net (gain) loss           949,000,000 882,000,000  
Regulatory amortization           0 0  
Regulatory asset           955,000,000 890,000,000  
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 870,000,000   890,000,000 870,000,000 870,000,000 955,000,000 890,000,000 870,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     890,000,000 870,000,000        
Reclassification adjustments     (55,000,000) (44,000,000)        
Total other comprehensive income (loss)     65,000,000 20,000,000        
Ending balance 870,000,000   955,000,000 890,000,000 870,000,000      
Alabama Power | Pension plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (120,000,000) (64,000,000)        
Reclassification adjustments     (54,000,000) (42,000,000)        
Alabama Power | Pension plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     (1,000,000) (2,000,000)        
Alabama Power | Pension plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0 0        
Alabama Power | Pension plans | Dispositions | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Alabama Power | Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Alabama Power | Other postretirement benefit plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           8,000,000 11,000,000  
Net (gain) loss           (16,000,000) 45,000,000  
Regulatory amortization           0 0  
Regulatory asset             56,000,000  
Regulatory liabilities           (8,000,000)    
Alabama Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 76,000,000   56,000,000 76,000,000 76,000,000 (9,000,000) 56,000,000 76,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     56,000,000 76,000,000        
Reclassification adjustments     (5,000,000) (5,000,000)        
Total other comprehensive income (loss)     (65,000,000) (20,000,000)        
Ending balance 76,000,000   (9,000,000) 56,000,000 76,000,000      
Alabama Power | Other postretirement benefit plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     60,000,000 15,000,000        
Reclassification adjustments     (1,000,000) (1,000,000)        
Alabama Power | Other postretirement benefit plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     (4,000,000) (4,000,000)        
Alabama Power | Other postretirement benefit plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0          
Georgia Power                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 11,356,000,000 10,719,000,000 11,931,000,000 11,356,000,000 10,719,000,000 14,323,000,000 11,931,000,000 11,356,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance   10,719,000,000 11,931,000,000 11,356,000,000 10,719,000,000      
Total other comprehensive income (loss)     3,000,000 3,000,000 2,000,000      
Ending balance 11,356,000,000   14,323,000,000 11,931,000,000 11,356,000,000      
Georgia Power | Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Georgia Power | Pension plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           12,000,000 14,000,000  
Net (gain) loss           1,218,000,000 1,091,000,000  
Regulatory amortization           0 0  
Regulatory asset           1,230,000,000 1,105,000,000  
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 1,129,000,000   1,105,000,000 1,129,000,000 1,129,000,000 1,230,000,000 1,105,000,000 1,129,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     1,105,000,000 1,129,000,000        
Reclassification adjustments     (71,000,000) (60,000,000)        
Total other comprehensive income (loss)     125,000,000 (24,000,000)        
Ending balance 1,129,000,000   1,230,000,000 1,105,000,000 1,129,000,000      
Georgia Power | Pension plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (196,000,000) (36,000,000)        
Reclassification adjustments     (69,000,000) (57,000,000)        
Georgia Power | Pension plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     (2,000,000) (3,000,000)        
Georgia Power | Pension plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0 0        
Georgia Power | Pension plans | Dispositions | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Georgia Power | Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Georgia Power | Other postretirement benefit plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           4,000,000 5,000,000  
Net (gain) loss           56,000,000 197,000,000  
Regulatory amortization           0 0  
Regulatory asset           60,000,000 202,000,000  
Georgia Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 213,000,000   202,000,000 213,000,000 213,000,000 60,000,000 202,000,000 213,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     202,000,000 213,000,000        
Reclassification adjustments     (10,000,000) (9,000,000)        
Total other comprehensive income (loss)     (142,000,000) (11,000,000)        
Ending balance 213,000,000   60,000,000 202,000,000 213,000,000      
Georgia Power | Other postretirement benefit plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     132,000,000 2,000,000        
Reclassification adjustments     (9,000,000) (8,000,000)        
Georgia Power | Other postretirement benefit plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     (1,000,000) (1,000,000)        
Georgia Power | Other postretirement benefit plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0          
Mississippi Power                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 2,943,000,000 2,359,000,000 1,358,000,000 2,943,000,000 2,359,000,000 1,609,000,000 1,358,000,000 2,943,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance   2,359,000,000 1,358,000,000 2,943,000,000 2,359,000,000      
Total other comprehensive income (loss)     0 0 2,000,000      
Ending balance 2,943,000,000   1,609,000,000 1,358,000,000 2,943,000,000      
Mississippi Power | Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Mississippi Power | Pension plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           2,000,000 3,000,000  
Net (gain) loss           165,000,000 155,000,000  
Regulatory amortization           0 0  
Regulatory asset           167,000,000 158,000,000  
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 154,000,000   158,000,000 154,000,000 154,000,000 167,000,000 158,000,000 154,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     158,000,000 154,000,000        
Reclassification adjustments     (10,000,000) (8,000,000)        
Total other comprehensive income (loss)     9,000,000 4,000,000        
Ending balance 154,000,000   167,000,000 158,000,000 154,000,000      
Mississippi Power | Pension plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (19,000,000) (12,000,000)        
Reclassification adjustments     (10,000,000) (7,000,000)        
Mississippi Power | Pension plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     0 (1,000,000)        
Mississippi Power | Pension plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0 0        
Mississippi Power | Pension plans | Dispositions | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Mississippi Power | Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
AOCI           0 0  
Mississippi Power | Other postretirement benefit plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           0 0  
Net (gain) loss           4,000,000 17,000,000  
Regulatory amortization           0 0  
Regulatory asset           4,000,000 17,000,000  
Mississippi Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 19,000,000   17,000,000 19,000,000 19,000,000 4,000,000 17,000,000 19,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     17,000,000 19,000,000        
Reclassification adjustments     (1,000,000) (1,000,000)        
Total other comprehensive income (loss)     (13,000,000) (2,000,000)        
Ending balance 19,000,000   4,000,000 17,000,000 19,000,000      
Mississippi Power | Other postretirement benefit plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     12,000,000 1,000,000        
Reclassification adjustments     (1,000,000) (1,000,000)        
Mississippi Power | Other postretirement benefit plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0 0        
Reclassification adjustments     0 0        
Mississippi Power | Other postretirement benefit plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0          
Southern Power                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 5,675,000,000 3,264,000,000 6,498,000,000 5,675,000,000 3,264,000,000 7,284,000,000 6,498,000,000 5,675,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance   3,264,000,000 6,498,000,000 5,675,000,000 3,264,000,000      
Total other comprehensive income (loss)     14,000,000 (10,000,000) 31,000,000      
Ending balance 5,675,000,000   7,284,000,000 6,498,000,000 5,675,000,000      
Southern Power | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity     (27,000,000) (27,000,000)   (20,000,000) (27,000,000)  
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     (27,000,000)          
Total other comprehensive income (loss)     7,000,000          
Ending balance     (20,000,000) (27,000,000)        
Southern Power | Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           0 1,000,000  
Net (gain) loss           26,000,000 32,000,000  
AOCI           26,000,000 33,000,000  
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     (2,000,000)          
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 0   33,000,000 0 0 26,000,000 33,000,000 0
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     33,000,000 0        
Reclassification adjustments       0        
Total other comprehensive income (loss)     (7,000,000) 33,000,000        
Ending balance 0   26,000,000 33,000,000 0      
Southern Power | Pension plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     5,000,000 0        
Reclassification adjustments     (2,000,000) 0        
Southern Power | Pension plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments       0        
Southern Power | Pension plans | Dispositions                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Southern Power | Pension plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       (33,000,000)        
Southern Power | Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           0 0  
Net (gain) loss           1,000,000 3,000,000  
AOCI           1,000,000 3,000,000  
Southern Power | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 0   3,000,000 0 0 1,000,000 3,000,000 0
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     3,000,000 0        
Total other comprehensive income (loss)     (2,000,000) 3,000,000        
Ending balance 0   1,000,000 3,000,000 0      
Southern Power | Other postretirement benefit plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (2,000,000) 0        
Southern Power | Other postretirement benefit plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     0          
Southern Power | Other postretirement benefit plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       3,000,000        
Southern Company Gas                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 8,001,000,000 8,001,000,000 9,022,000,000 9,109,000,000 9,109,000,000 8,570,000,000 9,022,000,000 9,109,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance 8,001,000,000   9,022,000,000 9,109,000,000        
Total other comprehensive income (loss) 26,000,000   2,000,000 (5,000,000)        
Ending balance 9,109,000,000 $ 8,001,000,000 8,570,000,000 9,022,000,000 9,109,000,000      
Southern Company Gas | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity     26,000,000 26,000,000   29,000,000 26,000,000  
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     26,000,000          
Total other comprehensive income (loss)     (2,000,000)          
Ending balance     29,000,000 26,000,000        
Southern Company Gas | Pension plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           (6,000,000) 0  
Net (gain) loss           (38,000,000) (42,000,000)  
AOCI           (44,000,000) (42,000,000)  
Regulatory asset               268,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     0          
Southern Company Gas | Pension plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           (17,000,000) (20,000,000)  
Net (gain) loss           83,000,000 197,000,000  
Regulatory amortization           94,000,000 40,000,000  
Regulatory asset           160,000,000 217,000,000  
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity (43,000,000)   (42,000,000) (43,000,000) (43,000,000) (44,000,000) (42,000,000) (43,000,000)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     (42,000,000) (43,000,000)        
Reclassification adjustments       0        
Total other comprehensive income (loss)     (2,000,000) 1,000,000        
Ending balance (43,000,000)   (44,000,000) (42,000,000) (43,000,000)      
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 267,000,000   217,000,000 267,000,000 267,000,000 160,000,000 217,000,000 267,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     217,000,000 267,000,000        
Reclassification adjustments     (25,000,000) (19,000,000)        
Total other comprehensive income (loss)     (57,000,000) (50,000,000)        
Ending balance 267,000,000   160,000,000 217,000,000 267,000,000      
Southern Company Gas | Pension plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (6,000,000) (1,000,000)        
Reclassification adjustments     0 0        
Southern Company Gas | Pension plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (20,000,000) 31,000,000        
Reclassification adjustments     (12,000,000) (18,000,000)        
Southern Company Gas | Pension plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments       0        
Southern Company Gas | Pension plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     18,000,000 0        
Reclassification adjustments     2,000,000 0        
Southern Company Gas | Pension plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     (15,000,000) (1,000,000)        
Southern Company Gas | Pension plans | Dispositions                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     8,000,000          
Southern Company Gas | Pension plans | Dispositions | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     34,000,000          
Southern Company Gas | Pension plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       0        
Southern Company Gas | Other postretirement benefit plans                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           1,000,000 0  
Net (gain) loss           (5,000,000) (3,000,000)  
AOCI           (4,000,000) (3,000,000)  
Regulatory asset               57,000,000
Southern Company Gas | Other postretirement benefit plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Prior service cost           2,000,000 (7,000,000)  
Net (gain) loss           (43,000,000) 47,000,000  
Regulatory amortization           37,000,000 6,000,000  
Regulatory asset             46,000,000  
Regulatory liabilities           (4,000,000)    
Southern Company Gas | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity (3,000,000)   (3,000,000) (3,000,000) (3,000,000) (4,000,000) (3,000,000) (3,000,000)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     (3,000,000) (3,000,000)        
Total other comprehensive income (loss)     (1,000,000) 0        
Ending balance (3,000,000)   (4,000,000) (3,000,000) (3,000,000)      
Southern Company Gas | Other postretirement benefit plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets                
Defined Benefit Plan Disclosure [Line Items]                
Total Stockholders' Equity 52,000,000   46,000,000 52,000,000 52,000,000 $ (4,000,000) $ 46,000,000 $ 52,000,000
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Beginning balance     46,000,000 52,000,000        
Reclassification adjustments     (6,000,000) (1,000,000)        
Total other comprehensive income (loss)     (50,000,000) (6,000,000)        
Ending balance $ 52,000,000   (4,000,000) 46,000,000 $ 52,000,000      
Southern Company Gas | Other postretirement benefit plans | Net (gain) loss                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     (2,000,000) (1,000,000)        
Southern Company Gas | Other postretirement benefit plans | Net (gain) loss | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     42,000,000 5,000,000        
Reclassification adjustments     0 (4,000,000)        
Southern Company Gas | Other postretirement benefit plans | Change in prior service costs                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     1,000,000          
Southern Company Gas | Other postretirement benefit plans | Change in prior service costs | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss     2,000,000 0        
Reclassification adjustments     0 3,000,000        
Southern Company Gas | Other postretirement benefit plans | Amortization of regulatory assets | Regulatory Assets                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Reclassification adjustments     $ (6,000,000)          
Southern Company Gas | Other postretirement benefit plans | Change from employee transfer                
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]                
Other comprehensive (income) loss       $ 1,000,000