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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]  
Summary of Valuation Allowances
Valuation Allowances
 
Georgia
Power
 
Mississippi Power
 
Southern Company Gas
 
Southern Company
 
(in millions)
Federal
$
6

 
$
—

 
$
11

 
$
19

State (net of federal benefit)
33

 
124

 
1

 
171

Balance at September 30, 2018
$
39

 
$
124

 
$
12

 
$
190