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Income Taxes (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     $ 81 $ 21
Effective tax rate (28.60%) 29.40%    
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Unrecognized tax benefits as of December 31, 2016 $ 484      
Tax positions from current periods 10      
Tax positions from prior periods 7      
Balance as of June 30, 2017 501      
Tax positions impacting the effective tax rate     37 20
Tax positions not impacting the effective tax rate     464 464
Balance of unrecognized tax benefits $ 484   501 484
Significantly increase (decrease) in amount of unrecognized tax benefits 12 months      
Kemper IGCC        
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Balance as of June 30, 2017 $ 464      
Balance of unrecognized tax benefits $ 464   464  
Unrecognized tax benefit, associated interest     36  
SOUTHERN Co GAS        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     19  
MISSISSIPPI POWER CO        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     46  
Effective tax rate (30.50%) (208.10%)    
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Unrecognized tax benefits as of December 31, 2016 $ 465      
Tax positions from current periods 3      
Tax positions from prior periods 0      
Balance as of June 30, 2017 468      
Tax positions impacting the effective tax rate     4  
Tax positions not impacting the effective tax rate     464  
Balance of unrecognized tax benefits 465   468 465
MISSISSIPPI POWER CO | Kemper IGCC        
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Balance as of June 30, 2017 464      
Balance of unrecognized tax benefits $ 464   464  
Unrecognized tax benefit, associated interest     36  
SOUTHERN POWER CO        
Income Tax Contingency [Line Items]        
Effective tax rate (114.70%) (74.00%)    
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Unrecognized tax benefits as of December 31, 2016 $ 17      
Tax positions from current periods 1      
Tax positions from prior periods 1      
Balance as of June 30, 2017 19      
Tax positions impacting the effective tax rate     19  
Tax positions not impacting the effective tax rate     0  
Balance of unrecognized tax benefits $ 17   19 17
Investment Tax Credit Carryforward        
Income Tax Contingency [Line Items]        
Tax credit carryforward, amount     1,900 $ 1,800
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Tax positions not impacting the effective tax rate     98  
Federal        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     18  
Federal | SOUTHERN Co GAS        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     18  
Federal | MISSISSIPPI POWER CO        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     0  
State (net of federal benefit)        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     63  
State (net of federal benefit) | SOUTHERN Co GAS        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     1  
State (net of federal benefit) | MISSISSIPPI POWER CO        
Income Tax Contingency [Line Items]        
Deferred tax assets, valuation allowance     $ 46