Fair Value Measurements - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	$ 48,151	$ 45,080
Fair Value	51,348	46,286
ALABAMA POWER CO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	7,625	7,092
Fair Value	8,305	7,544
GEORGIA POWER CO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	11,777	10,516
Fair Value	12,531	11,034
GULF POWER CO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	1,285	1,074
Fair Value	1,334	1,097
MISSISSIPPI POWER CO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	2,086	2,979
Fair Value	2,076	2,922
SOUTHERN POWER CO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	5,841	5,628
Fair Value	6,079	5,691
SOUTHERN Co GAS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	6,048
Fair Value	$ 6,471
Predecessor | SOUTHERN Co GAS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount		5,281
Fair Value		$ 5,491