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Fair Value Measurements - Schedule of Increase (Decrease) In Fair Value Of Funds (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]        
Increase (decrease) in fair value of funds $ 58 $ 50 $ 68 $ 168
ALABAMA POWER CO        
Fair Value, Option, Quantitative Disclosures [Line Items]        
Increase (decrease) in fair value of funds 39 25 49 87
GEORGIA POWER CO        
Fair Value, Option, Quantitative Disclosures [Line Items]        
Increase (decrease) in fair value of funds $ 19 $ 25 $ 19 $ 81