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Common Stock and Stock Compensation - Textual Performance Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate intrinsic value for options exercisable     $ 0    
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
Total compensation cost for award recognized in income, tax benefit     $ 4    
EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     100.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.21 $ 48.87 $ 47.75
ROE-based Portion Of The Stock Compensation Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of total performance share units granted     25.00% 25.00% 25.00%
Share based compensation, target grant fair value     20.00%    
EPS-based Portion Of The Performance Share Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of total performance share units granted     20.00% 25.00% 25.00%
Share based compensation, target grant fair value     20.00%    
TSR-based Portion Of The Stock Compensation Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of total performance share units granted     30.00% 50.00% 50.00%
Share based compensation, target grant fair value     30.00%    
EPS-based Portion Of The Stock Compensation Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of total performance share units granted     20.00% 25.00% 25.00%
Restricted Stock Portion Of The Stock Compensation Plan [Member] [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of total performance share units granted     30.00%    
Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share based compensation, target grant fair value     30.00%    
Performance share units, unvested (in shares)     700,000    
Equity instrument, granted (in shares)     600,000    
Equity instruments, vested (in shares)     100,000    
Total compensation cost for award recognized in income     $ 25    
Total compensation cost for award recognized in income, tax benefit     10    
Total unrecognized compensation cost related to award     $ 8    
Total unrecognized compensation cost related to award, weighted average period     13 months    
Share Based Compensation Arrangement by Share Based Payment Award Award Expiration Period     3 years    
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
Initial assumed percentage payout at end of performance period     100.00%    
Performance share units, unvested (in shares) 3,240,000.00   2,890,000.00 3,240,000.00  
Equity instrument, granted (in shares)     1,220,000.00    
Equity instruments, vested (in shares)     1,500,000.00    
Total compensation cost for award recognized in income     $ 74 $ 96 $ 88
Total compensation cost for award recognized in income, tax benefit     29 $ 37 $ 34
Total unrecognized compensation cost related to award     $ 30    
Total unrecognized compensation cost related to award, weighted average period     21 months    
ALABAMA POWER CO | EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     100.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.21 $ 48.86 $ 47.78
ALABAMA POWER CO | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.21    
Equity instrument, granted (in shares)     58,001    
Total compensation cost for award recognized in income     $ 3    
Total compensation cost for award recognized in income, tax benefit     $ 1    
ALABAMA POWER CO | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.07 $ 45.15 $ 46.42
Equity instrument, granted (in shares)     135,502 249,065 214,709
Total compensation cost for award recognized in income     $ 9 $ 15 $ 13
Total compensation cost for award recognized in income, tax benefit     4 $ 6 $ 5
Total unrecognized compensation cost related to award     $ 2    
Total unrecognized compensation cost related to award, weighted average period     21 months    
SOUTHERN Co GAS | EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.22    
SOUTHERN Co GAS | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.23    
Equity instrument, granted (in shares)   25,166 109,969 47,546  
Total compensation cost for award recognized in income     $ 4    
Total compensation cost for award recognized in income, tax benefit     2    
Compensation not yet recognized     $ 1    
Total unrecognized compensation cost related to award, weighted average period     13 months    
Award requisite service period     3 years    
SOUTHERN Co GAS | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.27    
Equity instrument, granted (in shares)     300,000    
Total compensation cost for award recognized in income     $ 8    
Total compensation cost for award recognized in income, tax benefit     3    
Total unrecognized compensation cost related to award     $ 6    
Total unrecognized compensation cost related to award, weighted average period     21 months    
SOUTHERN Co GAS | Restricted Stock Units (RSUs), Pre-acquisition [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total compensation cost for award recognized in income $ 13   $ 8    
Total compensation cost for award recognized in income, tax benefit $ 4   4    
Total unrecognized compensation cost related to award     $ 3    
Total unrecognized compensation cost related to award, weighted average period     12 months    
GEORGIA POWER CO          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
GEORGIA POWER CO | EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     100.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.22 $ 48.84 $ 47.78
GEORGIA POWER CO | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.22    
Equity instrument, granted (in shares)     59,218    
Total compensation cost for award recognized in income     $ 3    
Total compensation cost for award recognized in income, tax benefit     1    
Total unrecognized compensation cost related to award     $ 1    
Total unrecognized compensation cost related to award, weighted average period     13 months    
GEORGIA POWER CO | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.27 $ 45.17 $ 46.41
Equity instrument, granted (in shares)     138,102 261,434 236,804
Total compensation cost for award recognized in income     $ 10 $ 15 $ 15
Total compensation cost for award recognized in income, tax benefit     4 $ 6 $ 6
Total unrecognized compensation cost related to award     $ 3    
Total unrecognized compensation cost related to award, weighted average period     21 months    
GULF POWER CO          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
GULF POWER CO | EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     100.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.18 $ 48.83 $ 47.75
GULF POWER CO | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 48.88    
Equity instrument, granted (in shares)     15,736    
GULF POWER CO | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument, granted (in shares)     28,423 57,333 48,962
Fair value assumptions weighted average grant date fair value (in dollars per share)     $ 47.30 $ 45.18 $ 46.38
MISSISSIPPI POWER CO          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
MISSISSIPPI POWER CO | EPS-based and ROE-based Performance Share Units [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     100.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.22 48.84 47.77
MISSISSIPPI POWER CO | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.22    
Equity instrument, granted (in shares)     13,260    
MISSISSIPPI POWER CO | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share)     $ 49.24 $ 45.17 $ 46.41
Equity instrument, granted (in shares)     30,933 62,435 53,909
Total compensation cost for award recognized in income     $ 2 $ 4 $ 4
Total compensation cost for award recognized in income, tax benefit     $ 1 $ 1 $ 2
Minimum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     0.00%    
Maximum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Initial assumed percentage payout at end of performance period     200.00%    
Parent Company | Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Equity instrument granted in period, weighted average grant date fair value (in dollars per share) $ 53.83   $ 49.25    
Equity instrument, granted (in shares) 742,461        
Total compensation cost for award recognized in income     $ 12